Biocomposite Materials: Design and Mechanical Properties Characterization [1 ed.] 9789813340909, 9789813340916

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Composites Science and Technology

Mohamed Thariq Hameed Sultan Mohd Shukry Abdul Majid Mohd Ridzuan Mohd Jamir Azwan Iskandar Azmi Naheed Saba Editors

Biocomposite Materials Design and Mechanical Properties Characterization

Composites Science and Technology Series Editor Mohammad Jawaid, Lab of Biocomposite Technology, Universiti Putra Malaysia, INTROP, Serdang, Malaysia

Composites Science and Technology (CST) book series publishes the latest developments in the field of composite science and technology. It aims to publish cutting edge research monographs (both edited and authored volumes) comprehensively covering topics shown below: • Composites from agricultural biomass/natural fibres include conventional composites-Plywood/MDF/Fiberboard • Fabrication of Composites/conventional composites from biomass and natural fibers • Utilization of biomass in polymer composites • Wood, and Wood based materials • Chemistry and biology of Composites and Biocomposites • Modelling of damage of Composites and Biocomposites • Failure Analysis of Composites and Biocomposites • Structural Health Monitoring of Composites and Biocomposites • Durability of Composites and Biocomposites • Biodegradability of Composites and Biocomposites • Thermal properties of Composites and Biocomposites • Flammability of Composites and Biocomposites • Tribology of Composites and Biocomposites • Bionanocomposites and Nanocomposites • Applications of Composites, and Biocomposites To submit a proposal for a research monograph or have further inquries, please contact springer editor, Ramesh Premnath ([email protected]).

More information about this series at http://www.springer.com/series/16333

Mohamed Thariq Hameed Sultan Mohd Shukry Abdul Majid Mohd Ridzuan Mohd Jamir Azwan Iskandar Azmi Naheed Saba •

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Editors

Biocomposite Materials Design and Mechanical Properties Characterization

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Editors Mohamed Thariq Hameed Sultan Department of Aerospace Engineering, Faculty of Engineering Universiti Putra Malaysia Serdang, Selangor, Malaysia

Mohd Shukry Abdul Majid Faculty of Mechanical Engineering Technology Universiti Malaysia Perlis (UniMAP) Arau, Perlis, Malaysia

Mohd Ridzuan Mohd Jamir Faculty of Mechanical Engineering Technology Universiti Malaysia Perlis (UniMAP) Arau, Perlis, Malaysia

Azwan Iskandar Azmi Faculty of Mechanical Engineering Technology Universiti Malaysia Perlis (UniMAP) Arau, Perlis, Malaysia

Naheed Saba Laboratory of Biocomposite Technology, Institute of Tropical Forestry and Forest Products (INTROP) Universiti Putra Malaysia Serdang, Selangor, Malaysia

Composites Science and Technology ISBN 978-981-33-4090-9 ISBN 978-981-33-4091-6 https://doi.org/10.1007/978-981-33-4091-6

(eBook)

© Springer Nature Singapore Pte Ltd. 2021 This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Singapore Pte Ltd. The registered company address is: 152 Beach Road, #21-01/04 Gateway East, Singapore 189721, Singapore

Preface

The use of biocomposite materials has received significant attention in recent years, in particular, due to the proliferated awareness of and drive towards more environmentally sustainable technologies. In many cases, bio-based materials offer lightweight applications, added functionality such as damping/impact absorption and occupational health benefits. Thus bio-based materials have become a pressing issue both for academia and industry. A significant market driver for high volume applications is the potential to disassociate material costs from the inconsistent price of oil and energy. This book chapter offers ranges of biocomposites, and the natural fibres, bio-based polymers and bio-based core materials used to produce them and present the current best practice in design, mechanical characterisation and applications. The term “biocomposite” referred in the book represent fibre-reinforced polymer composite materials where the fibres and matrix are bio-based. Kenaf, Jute, Pineapple leaves (PALF), Hemp, and coir are frequent natural fibre reinforcements in biocomposites and have excellent mechanical properties. Loose fibre, non-woven mats and woven fabrics are the most common forms of natural fibre for composites, with aligned variants offering the best mechanical properties. Fibre treatments such as salinisation or acetylation can be used to reduce moisture uptake and advance compatibility with polymers. Artificial bio-based fibre reinforcements, such as regenerated cellulose, are also available and offer higher homogeneity. In addition to the studies aimed at refining basic mechanical properties and functionalities of the materials, other studies dedicated to developing eco-friendly composites and biomedical materials for use in tissue engineering, scaffolds and implants.

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We are thankful to all authors who contributed their valuable research in our edited book. Editors are also thankful to Springer team for supporting and cooperation during the whole project. Though the progress of the book has been delayed by the COVID-19 pandemic that swept the world, we have made through, and it is a true testament to the perseverance of all authors involved in the project. Thanks all. Serdang, Malaysia Arau, Malaysia Arau, Malaysia Arau, Malaysia Kuala Lumpur, Malaysia

Mohamed Thariq Hameed Sultan Mohd Shukry Abdul Majid Mohd Ridzuan Mohd Jamir Azwan Iskandar Azmi Naheed Saba

Contents

Green Biocomposites for Packaging Applications . . . . . . . . . . . . . . . . . . Florencia Versino, Olivia Valeria López, and María Alejandra García Towards Sustainable Buildings with Free-Form Geometries: Development and Application of Flexible NFRP in Load-Bearing Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hanaa Dahy PLA Hybrid Composites Reinforced with Nanomaterials . . . . . . . . . . . . Narendra Reddy

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Effect of Cellulose Nanocrystals on the Mechanical Properties of Polymeric Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Matthew J. Dunlop, Bishnu Acharya, and Rabin Bissessur

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Lignocellulosic Fiber-Reinforced PLA Green Composites: Effects of Chemical Fiber Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . Farkhondeh Hemmati, Tara Farizeh, and Jamshid Mohammadi-Roshandeh

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3D Printing of Continuous Natural Fibre Reinforced Biocomposites for Structural Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205 A. Mugeshwaran, Nalini Ranganathan, R. Joseph Bensingh, and Sanjay K. Nayak Manufacturing Defects in Natural Fibre Composites . . . . . . . . . . . . . . . 219 Suriani Mat Jusoh Carbon-Based Materials Reinforced Ultrahigh Molecular Weight Polyethylene and Biocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 229 Shahira Liza and Nur Hidayah Shahemi Water Absorption Properties of Natural Fibres Reinforced PLA Bio-Composite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 251 Rozyanty Rahman and Nur Rusyidah Mustapa

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Characterization of the Time of Phytosanitary Treatment of Frozen or Unfrozen Wood by Microwaves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 273 F. Erchiqui and H. Kaddami Synthesis, Characterization, in Vitro Biocompatibility and Antibacterial Properties Study of Nanocomposite Materials . . . . . . 307 Che Wan Sharifah Robiah Mohamad Enhancement of Local Drug Delivery System Using Different Design of Gentamicin Loaded in Carbonate Apatite Scaffold . . . . . . . . . . . . . . 321 N. Mamat, M. Mariatti, Z. A. A. Hamid, and B. H. Yahaya

About the Editors

Prof. Ir. Ts. Dr. Mohamed Thariq Hameed Sultan is a Professional Engineer (PEng) registered under the Board of Engineers Malaysia (BEM), a Professional Technologist (PTech) registered under the Malaysian Board of Technologists, and also a Charted Engineer (CEng) registered with the Institution of Mechanical Engineers United Kingdom, currently attached to the Universiti Putra Malaysia as the Head of the Biocomposite Technology Laboratory, Institute of Tropical Forestry and Forest Products (INTROP), UPM Serdang, Selangor, Malaysia. Being the Head of the Biocomposite Technology Laboratory, he is also appointed as an Independent Scientific Advisor to Aerospace Malaysia Innovation Centre (AMIC) based in Cyberjaya, Selangor, Malaysia. He received his Ph.D. from the University of Sheffield, UK. He has about 10 years of experience in teaching as well as in research. His area of research interests includes hybrid composites, advance materials, structural health monitoring, and impact studies. So far he has published more than 100 international journal papers and received many awards locally and internationally. In December 2017, he was awarded a Leaders in Innovation Fellowship (LIF) by the Royal Academy of Engineering (Raeng), UK. He is also the Honourable Secretary of the Malaysian Society of Structural Health Monitoring (MSSHM) based in UPM Serdang, Selangor, Malaysia. Currently, he is also attached to the Institution of Engineers Malaysia (IEM) as the Deputy Chairman in the Engineering Education Technical Division (E2TD). Dr. Mohd Shukry Abdul Majid received his Bachelor of Engineering in Mechanical Engineering from University Manchester Institute of Science and Technology (UMIST) in 2001. Upon his return to Malaysia, he worked as a research and development (R&D) engineer at a semiconductor industry before joining Universiti Malaysia Perlis (UniMAP) as a lecturer in 2004. He completed his M.Sc. in Mechanical Systems Engineering from the University of Liverpool in 2005 and his Ph.D. in Composite Engineering from Newcastle University, UK in 2011. Currently, he is serving Universiti Malaysia Perlis as an Associate Professor at Faculty of Mechanical Engineering Technology. An advocate of interdisciplinary research, his research interests lie in the strength of material’s area with emphasis ix

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on the composite piping, looking at the performance of composite structures, NDE’s of composites and natural fibre/green composites, hybrid reinforced/filled polymer composites, lignocellulosic reinforced/filled polymer and biodegradable composites. His aptitude for high-quality research of international standing has been supported by his 188 Scopus indexed publications with 47 (32 Q1) publications in ISI-ranked Journals having a cumulative impact factor (CIF) of 148.948. He received his professional engineer qualification (Ir.) from Board of Engineer Malaysia (BEM) in Mac 2016 and has been a Chartered Engineer (CEng) from the Engineering Council, UK since 2014. Dr. Mohd Shukry has been honoured with numerous local and international awards for his achievements. He is the first recipient from Technical University Network (MTUN) to have been awarded Malaysia’s Research Star Award 2017, as one of Malaysia’s most promising and influential researchers by the Ministry of Higher Education Malaysia (MOHE). Dr. Mohd Ridzuan Mohd Jamir is a Professional Engineer (PEng) registered under the Board of Engineers Malaysia (BEM), a Professional Technologist (PTech) registered under the Malaysian Board of Technologists, and also a Chartered Engineer (CEng) registered with the Institution of Mechanical Engineers United Kingdom, currently attached to the Universiti Malaysia Perlis as an Associate Professor at Faculty of Mechanical Engineering Technology. He obtained Diploma in Mechanical Engineering from Universiti Teknologi Malaysia (UTM) in 2006. He graduated in Bachelor of Engineering (Mechanical) and Master of Engineering (Mechanical) from Universiti Teknologi Malaysia (UTM) in the year of 2009 and 2010, respectively. He also worked as Quality Assurance Engineer in Venture Pintarmas Sdn Bhd in Johor Baharu in 2010 before joined Universiti Malaysia Perlis (UniMAP) in the same year. He received his Ph.D. in Mechanical Engineering from Universiti Malaysia Perlis (UniMAP) in 2016. He has about 10 years of experience in teaching as well as in research. An advocate of interdisciplinary research, his research interests includes the strength of material’s area with emphasis on the natural fibre composite, looking at the performance of composite structures and tribological properties, hybrid reinforced/filled polymer composites, lignocellulosic reinforced/filled polymer and biodegradable composites. He has published more than 100 international journal research paper with Scopus index and ISI ranked journal publications. In addition, he is also a regular reviewer for high-impacts ISI ranked journals. Dr. Azwan Iskandar Azmi received his bachelor’s degree in Mechanical Engineering from Purdue University, USA in 1999 and Master’s in Advanced Manufacturing Technology from Universiti Teknologi Malaysia in 2003. In 2013, he completed his doctoral study at The University of Auckland, New Zealand in Mechanical Engineering with specialisation in the area of fibre reinforced composite machining. He is currently serving the Faculty of Mechanical Engineering Technology, Universiti Malaysia Perlis (UniMAP) as Associate Professor. Dr. Azwan is a Professional Engineer (PEng) registered under the Board of Engineers Malaysia (BEM), a Professional Technologist (PTech) registered under the

About the Editors

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Malaysian Board of Technologists (MBOT), and also a Chartered Engineer (CEng) registered with the Institution of Mechanical Engineers (IMechE) in the UK. He has more than 10 years of experience in teaching, research, and industries. His area of research interests includes machining and machinability of carbon, glass, and their hybrid composites. Currently, the research also covers the drilling and milling of lignocellulosic reinforced polymer composites. His interest extends on the machinability study of difficult-to-cut metal alloys such as titanium, Inconel, and nickel-titanium (NiTi) alloys. So far, he has more than 50 research articles in renowned international journals, proceedings and review papers. He has been awarded with research funds from Ministry of Higher Education Malaysia (MOHE) and Ministry of Science Technology and Innovation (MOSTI). Due to the research outputs, Dr. Azwan has served as technical reviewer for a number of reputable high impact ISI ranked journals and international conferences. Dr. Naheed Saba completed her Ph.D. (Nanocomposites) with Distinction from Laboratory of Biocomposites Technology, Institute of Tropical Forestry and Forest Products (INTROP), Universiti Putra Malaysia, Serdang, Selangor, Malaysia in 2017. She completed her masters in chemistry and also completed her postgraduate diploma in environment and sustainable development from India. She has published over 50 scientific Peer review Articles in International Journal and 4–5 Articles come under top cited articles in Construction and Building Materials, and Polymers Journal during 2016–2019. She edited six books from Elsevier and also published more than 25 book chapters in Springer, Elsevier, and Wiley publication. She attended few international conferences and presented research papers. Her research interest areas are nanocellulosic materials, fire-retardant materials, natural fiberereinforced polymer composites, biocomposites, hybrid composites, and nanocomposites. She is also recipient of International Graduate Research Fellowship and Graduate on time (GOT) award from Universiti Putra Malaysia, Malaysia. Presently she is Editor-in-Chief of The Journal of Composites and Advance Materials (ISSN 2716-8018) is a peer-reviewed bi-annual journal. She is also reviewer of several international journals such as Cellulose, Constructions and Building Materials, Composite Part A,Composite Part B, Journal of Polymers and The Environment, Journal of Energy Storage, Journal of Elastomers and Plastics, Journal of Materials Research and Technology, BioResources, and Carbohydrate Polymers. Her H-index = 17 (Scopus); H-Index = 20 (Google Scholar).

Green Biocomposites for Packaging Applications Florencia Versino, Olivia Valeria López, and María Alejandra García

Abstract In recent years, research on environmentally sustainable packaging has been gaining momentum, primarily driven by consumer ecological consciousness. Green biocomposites play an important role in novel and innovative materials for the emerging sustainable packaging industry, being intrinsically biobased and biodegradable. Therefore, the following chapter is aimed to make an overview of the main trends on green biocomposites study and development, their environmental impact and their importance in future production systems. A revision of the polymeric matrices and fillers most widely used for green biocomposites and bionanocomposites manufacture is done, and the results of the latest investigations on the subject are discussed. Besides, their role in active and intelligent packaging is reviewed as well as their implementation for 3D printing technologies. Finally, the relevance of these materials study and development in terms of environmental impact is herein considered, remarking the importance of adequate life cycle assessment of the developed green biocomposites in comparison to conventional materials used for similar packaging applications. Keywords Bioplastics · Packaging · Sustainable · Biodegradable · Biomass

F. Versino (B) · M. A. García Centro de Investigación y Desarrollo en Criotecnología de Alimentos (CIDCA), UNLP-CONICET-CICPBA, La Plata, Argentina e-mail: [email protected] URL: https://www.cidca.quimica.unlp.edu.ar O. V. López Planta Piloto de Ingeniería Química (PLAPIQUI), UNS-CONICET, Bahía Blanca, Argentina URL: https://www.plapiqui.conicet.gov.ar F. Versino Departamento de Ingeniería Química, Universidad Nacional del La Plata, La Plata, Argentina © Springer Nature Singapore Pte Ltd. 2021 M. T. Hameed Sultan et al. (eds.), Biocomposite Materials, Composites Science and Technology, https://doi.org/10.1007/978-981-33-4091-6_1

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1 Introduction Composite materials comprise a wide range of materials, which consist of at least two compounds that differ on their physical or chemical characteristics interacting in a way that leads to a single material with hopefully better properties than its components separately. Generally, composites are comprised by a polymeric continues phase (known as matrix) and a dispersed phase (known as reinforcement), though some are prepared by mixture of two or more polymers (Shekar and Ramachandra 2018). The vast amount of possible compounds combinations in composite materials development has driven these materials market growth in various fields of application, mainly automotive, marine, aerospace, and construction industries (Rodriguez et al. 2012; Ramamoorthy et al. 2019). They were first developed as a low-cost alternatives to pure components and as high-performance and low-weight materials. Most early studies, as well as current work, focus on biobased and biodegradable compounds to develop the herein called green biocomposites which are expected not only to be cost-effective but also more environmentally friendly. This type of materials have become of interest for their use in the packaging industry where conventional composites face some limitations: their waste management and recycling are difficult and still underdeveloped generating large non-biodegradable plastic waste, while contributing to petrochemical resources depletion (La Mantia and Morreale 2011; Shekar and Ramachandra 2018). Packaging materials must be sufficiently resistant to protect and preserve the product from production through transportation and storage until consumption, but are discarded usually with little to no reuse (Song et al. 2009). Hence the development of green biocomposites, mainly those which are not only biobased but also biodegradable, has been extensively studied. Petit et al. (2019) showed that despite of monetary savings there is a strong preference of consumers for smaller food packages, even individual portions, over larger packages because of anticipated food waste association. This proves, on the one hand, that consumers have a key role in market tendency building, thus waste management and environmental public education is of critical importance to move towards more sustainable production systems. On the other hand, smaller and portioned food packages prevent food waste, yet resulting in greater litter generation from packaging, therefore new packaging systems that could minimise both types of wastes need to be developed. In this that regard, biodegradable packaging plays a central role. Moreover, a cost-effective package should be the cheapest option that could preserve the products in good conditions until it is consumed in order to avoid products loses due to damage or expiration. In addition, packaging should be designed as to minimise raw materials and energy use in its production and reduce the energy consumption in transportation. Some alternatives oriented in this regard are, for instance, the thickness reduction on PET bottles design or the square profile design of bottles in order to optimise pallet space utilisation. The minimisation of raw materials use and products losses, as well as the reduction of transport volume and weight not only help reduce costs but also make a more efficient use of the valuable resources (Emblem 2012).

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Nowadays, thanks to the worldwide greater environmental awareness, leading consumer goods companies are committed to a more holistic approach to good production, packaging, and distribution in turn of more sustainable processes to avoid pollution and further resources depletion. Efficient and cost-effective production and design of packaging materials, involving raw materials use reduction and development of new biobased and biodegradable packaging materials, are very much part of this holistic worldview (Farmer 2013). Consequently, in comparison to petroleum-based products, sustainable biobased products may have benefits for all agents comprised: the producers and customers as well as the whole environment (Georgios et al. 2016). Briefly, in developing and designing sustainable alternatives for the packaging industry the whole process from cradle to grave should be considered. In the production stage, the type of raw material employed, its environmental impact with respect to resources consumption (water, earth, energy, etc.) during processing and transportation and the pollution that from its use is derived must be considered. During the shelf-life stage, its efficiency to preserve the specific packed-products properties as to prevent product loses, taking into account mechanical resistance, permeability and, eventually, active packaging properties should be contemplated. Lastly, in the endof-life stage, the possibility of materials reuse, recycling or finally energy recovery or composting (in the case of biodegradable materials) must be evaluated. There is still much study and work to do in this respect, yet there is a common objective of adapting our products and processes to the future requirements, tending toward more conscious and sustainable production. In general, the mechanical properties of composites are improved by fillers addition due to their higher strength and stiffness than those of polymers. An extensive revision and comparison of these properties has already been presented (Georgios et al. 2016; Castillo et al. 2017; Ramamoorthy et al. 2019). Therefore, this chapter aims to an extensive revision of the state-of-the-art on biobased and biodegradable composite materials investigations for packaging applications. On the one hand, the type of polymer matrices will be revised, especially its use in active packaging systems. On the other hand, different types and sizes of filler employed and processing techniques will be discussed. Various studies have gathered and compared different composites and green composites properties. Thus, even though a wide range of biocomposites properties will be discussed herein, it will certainly be incomplete owing to the versatility and numerous possibilities of formulations of composites materials comprising: polymer matrix type; filler type, size and form; additives type and content (such as plasticizer, cross-linking agents, among others); processing techniques employed; and processing conditions.

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2 Green Composites Formulations, Processing, Structure, and Final Properties 2.1 Towards Greener Composites As it was previously mentioned, polymer composites were first developed in order to reduce costs and weight. Besides, by these means there was also an implied reduction in conventional polymers use, which could contribute to decreasing petroleum reserves depletion and all environmental impact derived from its overuse. In addition, the use of basically recyclable polymers (mainly polyolefins), meaning that post-consumer plastics recovered from waste management systems could be partially used in place of virgin raw material, was a step forward greener or environmentally friendly composite material (La Mantia and Morreale 2011). A step further led to the substitution of mineral inorganic fillers (such as glass fibre) by natural-organic fillers (for instance, wood flour or fibres extracted from plants). The latter present several advantages from an ecological point of view: they are essentially biodegradable and renewable materials; they are usually derived from relatively abundant plants (often from wastes) and therefore very cheap; they are also more safe to process both for employees and processing equipment; they can be easily incinerated; in comparison to inorganic fillers lead to lighter composites; and allow obtaining composite materials with thermal and acoustic insulation (Joshi et al. 2004; Dittenber and GangaRao 2012; Sapuan et al. 2016; Castillo et al. 2017). Even though green composites containing novel sustainable natural fillers have drawn several researches for different industrial applications as a way of achieving a more efficient use of resources, some limitations regarding composites ductility, processability, and dimensional stability have arisen (La Mantia and Morreale 2011; Nayak and Khuntia 2019). These problems derive mostly from the hydrophilic nature of bioreinforcements and their low thermal stability. Numerous chemical and physical modifications of organic fillers can be performed in order to improve both fibre properties as well as fibre-polymer interaction, and consequently the composites final performance (Azwa et al. 2013; Shalwan and Yousif 2013; Castillo et al. 2017). Nonetheless, an integrally sustainable approach with real environmental impact can only be obtained by substituting conventional oil-based polymers by biobased and biodegradable ones. These, however, present new limitations regarding especially lower mechanical performance and dimensional stability. Thus, these materials should be studied and developed for certain applications where very high mechanical resistance and durability are not relevant properties, which is the case of most packaging uses. Researches on green biocomposites are currently focused on the selection of the most suitable biodegradable matrix for each type of product and the optimization of formulations and processing conditions. Much of these investigations are exploring novel sustainable natural fillers as bioreinforcement-agents, being agricultural waste or by-products key players in the search of circular production systems.

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As regards the biomaterials market situation, it can be stated that even though bioplastics have had a strong growth in the world market in the last decade, it is still in an opening phase. According to a report by the company Allied Analytics LLP, 4869 thousand tons of bioplastics were commercialized in 2014 and it is estimated that for the 2018–2024 period the annual growth rate is 20.45% (Allied Analytics LLP 2018). This growth is due not only to the various potential applications of these materials but also to favourable government policies aimed at increasing the production and consumption of environmentally friendly and sustainable products. Yet, significant research efforts are required to develop green biocomposites with similar performance to their petroleum-based counterparts being as well cost-competitive.

2.2 Polymer Matrices Before analysing and comparing different polymer matrices used in green biocomposites, a proper classification of polymers should be stated. Usually, it would be considered as green biocomposites such composites that necessarily consist of a bioplastic as polymeric phase. This term comprises all plastics that are either biodegradable or bio-based or both (Fig. 1), hence there are bio-based polymers that are not biodegradable (i.e. green polyolefins) or biodegradable materials that are petroleum-based (i.e. polycaprolactone). However, to the extent of this chapter will be considered for green composite constituent those that are necessarily biodegradable. As for non-biodegradable polymer matrices used in the field of green composites (which could be or not biobased), these are mainly polyolefins (as well as some polyurethanes or phenolic resins) in combination with natural fillers: mostly lignocellulosic fibres (Netravali and Chabba 2003; Iyer et al. 2015; Dey and Ray 2018; Dixit and Yadav 2019; Hidalgo-Salazar and Salinas 2019) or mineral fillers (Hatakeyema et al. 2005; Cazan et al. 2019; Hanken et al. 2019). The greener option in this case is the use of the biobased counterpart. As for biodegradable bioplastics from fossil origin, most composite works reported employ polycaprolactone (PCL) or polyvinyl alcohol (PVA) as polymer matrix and in some cases combined with other biobased biodegradables polymer, such as starch and chitosan (Liu et al. 2010; Ludueña et al. 2012; T˘anase et al. 2015; Xiong et al. 2018; Ahmed et al. 2019; Huang et al. 2019; Wang et al. 2019; Wu et al. 2019). Even though these polymer matrices usually have better mechanical properties and water resistance than natural polymers and, thanks to their biocompatibility, are more suitable for biomedical applications, they have a considerably higher cost (Chen et al. 2013; Trakoolwannachai et al. 2019; Zhang et al. 2019). Natural polymers and biodegradable bioplastics synthesised from biomass (i.e. polylactic acid (PLA) or polyhydroxyalkanoates (PHAs)) are the most desirable polymeric matrices from an environmental point of view, owing that culture practices are sustainable with controlled water usage and minimum or no agrochemical application. In comparison to conventional plastic, these bioplastics have various advantages: low-cost (in the case of natural biopolymers), great availability, and

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Fig. 1 Polymers classification according to biodegradability and source. Note PE: polyethylene; PP: polypropylene; PET: polyethylene terephthalate; PVC: polyvinyl chloride PS: polystyrene; PLA: polylactic acid; PBS: poly butylene succinate; PHA: polyhydroxyalkanoate; PHB: polyhydroxybutyrate; PHBV: Poly(3-hydroxybutyrate-co-3-hydroxyvalerate); MCC: microcrystalline cellulose; PCL: polycaprolactone; PVA: polyvinyl alcohol; PGA: polyglycolic acid

biodegradability (Ramamoorthy et al. 2019). The most widely applied renewable resources include plant oils, polysaccharides (primarily cellulose and starch), and proteins; most of them used as well as precursors for synthesised resin (Georgios et al. 2016). The major limitations for the application of these biopolymers are due to their lower thermal, water, and mechanical resistance. For instance, PLA is sensitive to hydrolytic degradation under melt processing condition in the presence of small amounts of moisture (Mukherjee and Kao 2011); and starch can only be thermo-processed in presence of plasticisers being unplasticised films extremely brittle (López et al. 2008; Versino et al. 2019). Besides, bioplastics production is not cost-competitive nowadays with commodity traditional polymers, not due to the raw materials cost but rather their low volume production (Georgios et al. 2016). The literature review show a vast amount of studies on biodegradable and biobased polymers, being these key to developing eco-friendly and sustainable packaging; PLA, (Mukherjee and Kao 2011; Arrieta et al. 2014; Fortunati et al. 2015; Yusoff et al. 2016; Piekarska et al. 2017; Komal et al. 2020), starch (Famá et al. 2009; Vercelheze et al. 2012; Bodirlau et al. 2013; Versino and García 2014; López et al. 2015; Merci

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et al. 2019) and their combination with other bioplastics (Kaisangsri et al. 2012, 2014; De Oliveira et al. 2019) are the most used matrices for green biocomposites.

2.3 Natural Fillers Natural fillers can be both organic and inorganic. In this regard, layered silicates (clays), carbon nanotubes, and mineral nanoparticles (talc), among others, are the most widely used inorganic fillers in composite materials (Castillo et al. 2017). These are usually nanosized fillers (meaning that at least one of their dimensions is smaller than 100 nm) that have proven to induce important changes in composites properties even at very low concentrations (Castillo et al. 2015). Even though these fillers are not biodegradable nor renewable, they are usually natural fillers and would be expected to be generally innocuous to the environment when the biocomposite is composted. Nonetheless, their toxicity is still under study (Yu et al. 2019). Organic fibres, on the other hand, are typically lignocellulosic materials derived from plant tissue, thus fully biodegradable and annually renewable. They are made of three principal components: cellulose, lignin, and hemicellulose. Cellulose is the most abundant compound of the fibres which is accountable for their inherent strength and stability, while hemicellulose contributes to maintain the structure of the fibre, and lignin protects the matrix from pathogen attack (Taiz and Zeiger 2002). Natural plant fibres are a more economic and environmentally friendly alternative to synthetic ones: they are organic renewable material with low energy process requirements, and therefore carbon dioxide neutral if composted (John and Thomas 2008). Moreover, they can be obtained from agricultural or wood wastes, contributing to a more sustainable material production (Versino et al. 2019). Additionally, they are less abrasive to processing equipment, essentially cheaper, and present lower density (Sapuan et al. 2016). Mohanty et al. (2000), Väisänen et al. (Väisänen et al. 2016), and Djafari Petroudy (2017) have extensively reviewed the composition and mechanical properties of the most industrially used natural fibres. Natural organic fibres density there reported vary from 0.7 to 1.5 gm/cm3 , significantly lower than that of glass fibre (2.5 gm/cm3 ). In general, the reinforcing efficiency of plant fibres depend on cellulose nature and crystallinity, and its alignment in the cell walls: high cellulose content and low microfibril angle (MFA, defined as the angle microfibrils make with respect to the fibre axis) are desirable (Georgios et al. 2016; Djafari Petroudy 2017). Comparatively, ramie, hemp and flax fibres showed the higher cellulose content and lower MFA and, correspondingly, the highest tensile strength and Young’s moduli, indicating superior mechanical resistance. Biofibres have a low thermal stability which limits both processing conditions and the recyclability of the composites containing them. If they are exposed to high processing temperatures (typically circa 200 °C) for a certain time, their mechanical properties can be significantly degraded. For instance, it has been reviewed a 10% drop on naked ramie fibres tensile strength with a 10 min exposure at 200 °C, while this effect was retarded in PP/sisal fibre composite signifying that the fibre

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was protected by the polymer matrix (Ramamoorthy et al. 2019). Moreover, Chaitanya et al. (2019) proved that there is a marked deterioration of PLA/sisal fibre biocomposites thermal stability with recycling cycles. Another major disadvantage of natural organic fillers is their hydrophilicity and moisture content. This affect their mechanical properties and dimensional stability and those of the composites that contain them, as well as the interfacial adhesion between the fillers and the polymer (generally hydrophobic) that has an important impact on their dispersion in the matrix (La Mantia and Morreale 2011). Therefore, research has been focused on the modification of natural fillers surface in order to improve polymer-filler interaction and fibres properties. Even when both components are essentially hydrophilic, the addition of the untreated filler could lead to poor mechanical properties in comparison to the neat polymer if filler particles agglomeration occurs. In this respect, both chemical and physical modifications of natural fibres have been widely studied and reported (Kalia et al. 2009; Kabir et al. 2012). The most used treatment is mercerization (alkali treatment) with dilute NaOH solution, a basic treatment in paper industry which partially removes lignin, waxes and oils from the fibres. This treatment distorts the packed crystalline cellulose structure, generating rougher fibre surface and more porous structure which improves wetting. Alkali modification is usually followed by fibre acetylation which helps stabilise cell walls and improve water resistance (increase hydrophobicity). La Mantia and Morreale (2011) have made a clear recount of the chemical modification used on natural fillers, remarking that treatments with TDI (reaction with toluene-2,4-diisocyanate), dicumylperoxide, and silanes appear to be the most effective in mechanical properties improvement, whereas mercerization and acetylation enhances thermal and dimensional stability. Ozone and plasma treatments have also been reported, indicating a greater wettability and roughness of the treated fibres, which enhanced polymer-filler interaction and therefore improved the mechanical properties of the composites obtained with different polymer matrices, both bio and conventional ones (De Oliveira et al. 2017; Ventura et al. 2017; Fazeli et al. 2019; Putra et al. 2020; Sánchez et al. 2020). However, various factors should be assessed when choosing fillers treatments for green composites: cost (especially in the case of complicated techniques which can make some methods impractical for industrial applications), energy consumption, type and amount of effluents generated, reagents toxicity in long term exposures (for safe work conditions), and the global environmental impact of the process. In this regard, plasma treatments are arising great interest since they have low energy and chemical reagents use, require no water and can be incorporated to continuous processes (Ventura et al. 2017). Another alternative to produce green composites with improved components interfacial adhesion and filler dispersion is the addition of small amounts of a third component which acts as compatibilizer between the filler and the polymer matrix by forming chemical bonds (either covalent or Van der Waals kind) with both parts (La Mantia and Morreale 2011). Lysine-diisocyanate and esterified lignin, for example, have been used as coupling agents in different green biocomposites reported in the literature (Lee and Wang 2006; Anugwom et al. 2019). The use of

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non-toxic and preferably biobased compatibilizer should be preferentially studied for new sustainable green biocomposites development. Yet one of the major problems in the use of natural-organic fillers is the variation of properties from botanical sources, harvesting seasons, and regions with different solar radiation, precipitations, soil composition, and minimum/maximum day temperatures. An attempt to address this problem is to mix batches of different harvest fibres of the same type or different type of fibres (Georgios et al. 2016).

3 Processing Biocomposites are manufactured using various conventional techniques for plastic processing, typically extrusion and later compression and injection moulding. For each composite formulation, processing parameters such as temperature, pressure, and time need to be studied to optimise the materials end properties. Likewise, not only the filler and resin intrinsic properties but also the filler-polymer interface characteristics are critical for the composite’s final properties (Shekar and Ramachandra 2018). The mayor limitations in green biocomposite processing is due to the hygroscopic nature of the fillers and their poor thermal resistance (La Mantia and Morreale 2011). The presence of moisture, as well as the use of water as plasticiser (especially in the case of starch-based materials), generally leads to the formation of water vapour during processing which can interfere with polymer filler adhesion, generating structural defects (i.e. voids) in the final product during moulding if a venting system is not provided. Consequently, natural fillers are usually dried to some extent (approx. 2–3%) to avoid fibre agglomeration during processing without compromising their mechanical performance (Singh et al. 2017). Processing techniques as well as composite formulation have proven to have an impact on the biocomposites final properties. An interesting work by Morreale et al. (2008) on MaterBi® based composites with wood flour, showed that filler content was the process variable which had the greater impact on the materials rigidity, while impact strength was significantly modified by mixing speed and the filler aspect ratio. Ashori and Nourbakhsh (2010) also concluded that aspect ratio had a greater effect on the mechanical properties of PP composites with oak and pine fibres than particle size. Many other authors have shown how comparatively different processing parameters have statistically greater impact over biocomposites properties (Dauda et al. 2007; Castillo et al. 2013; Muthuraj et al. 2016). Due to the relatively low thermal stability of natural-organic fillers, processing temperature conditions of green composites are limited, being 140–190 °C the reported typical processing temperature range for extrusion, blend-mixing and thermo-moulding of different polymer matrices (Morreale et al. 2008; López et al. 2015; Jumaidin et al. 2016; Mendes et al. 2016; Scaffaro et al. 2018; Versino et al. 2019). An electromagnetic induction system for PLA/jute fabric composite proposed by Tanaka et al. (2008) allows the instant heating of the mold surface, therefore reducing production cycle times, and consequently, process costs and possible

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thermal degradation of the composites components. The casting method of composite materials is generally performed at low temperatures and is widely employed in laboratory or small production scale studies, yet due to its high solvent use it is difficult to scale-up to industrial proportions. However, the study of filler-matrix in casting systems is simple and of great use to understand the composites components interactions. In addition, as the inclusion of fibres tends to increase the molten-polymer batter viscosity, higher shear forces are needed to make the filler-polymer mixtures flow. Therefore, the rheology study of the biocomposites formulations is of great importance in terms of processing conditions needed as well as its potential scaleup. Besides, the rheological responses are closely related to the fillers dispersion and orientation, as well as their interactions with the polymer matrix (Wang et al. 2018; Borchani et al. 2019). In general, rheological rotational studies show an increase in the viscosity of green biocomposites under melt flow conditions, indicating an overall hindrance of its processability (Scaffaro et al. 2018; Wang et al. 2018; Xiao et al. 2019). A similar trend was observed on corn TPS biocomposites during melt-mixing process, where an increase in the mixing maximum torque which is associated to the material viscosity during processing and hence to the melt resistance to flow (Vallejos et al. 2011; Versino et al. 2019). However, an opposite effect was observed when working on cassava TPS biocomposites with cassava roots by-products, which was attributed to a thermodynamic favourability of the diffusion process of plasticisers into cassava starch granules with filler addition (Versino et al. 2015), probably due to amylopectin content and chain length differences between starch botanical origins. Moreover, Scaffaro et al. (2018) and Xiao et al. (2019) observed a more marked shearthinning effect (decrease in apparent viscosity at higher oscillatory frequencies) on biocomposites with higher fillers loads (>20%) that indicate a greater interaction among particles, which is disrupted at higher frequencies by disentanglement and reorientation of the fillers and the polymer chains in the flow direction. Another interesting study carried out by Borchani et al. (2019) showed the impact of different alkali fibres treatments on Mater-Bi® based biocomposites on the rheological behaviour of the molten mixture, exhibiting a greater gel network strength of treated fibres biocomposites due to the greater interaction between the matrix and the treated filler. Rheological behaviour under melt flow conditions results of important design parameters also for biocomposites for 3D printing applications, an alternative which is also currently being widely studied. The development of green biocomposites 3Dprinting filaments may result of great interest for the packaging industry, allowing to enhance the package mechanical resistance compared to neat polymer filaments, as well as the possibility of designing specially featured packages to minimize products damage and package volume. Le Duigou et al. (2019) developed PLA/flax fibre filaments with better mechanical properties than many other natural fibres biocomposites.

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4 Packaging Applications Green composite materials have been studied and used in many industrial applications such as construction (windows frames, doors, insulating panels), furniture, railroad sleepers, automotive parts, gardening items, packaging, and various other applications of low cost and not high mechanical performance requirements (La Mantia and Morreale 2011; Gurunathan et al. 2015; Dey and Ray 2018). A possible greener and convenient trend under study is the use of recycled polymers instead of virgin ones in order to reduce production costs and develop more sustainable materials. Many shoes and clothing international brands have moved towards this eco-friendlier, yet biocomposite materials are not extensively employed. In this regard, packaging applications are the most promising scenario for green biocomposites use and development, since it represents 30–35% of the urban solid waste annually generated (Tencati et al. 2016), have mild mechanical resistance requirements, and are usually discarded after single use, especially for food-packaging where recycling is more complex and water consuming. Biodegradable biobased composites are one of the most promising sustainable alternatives for packaging applications. A review by Tang et al. (2012) summarizes the barrier and mechanical properties of the most typically studied biopolymer and biobased composites films for food packaging applications, remarking that poor mechanical and water vapour barrier properties represent the greatest limitations of these materials. However, new and enhanced materials have been developed to date, with strong focus on active and intelligent packaging. The first has an specific function different from the usual protective characteristic of the package that results from the presence or controlled-release of an active compound (i.e. antimicrobial activity, gas scavenging, etc.), while the latter provides information to the consumer regarding the products conditions based in some measurable quality parameter sensed by the packaging material (i.e. pH, temperature limits, etc.). Furthermore, the use of 3D printing of green biocomposites has also gained interest in the packaging industry. By this technology customised structures can be designed with structural optimised properties in order to minimise virgin materials use by reducing the thickness needed to achieve the same mechanical property and waste due to cuts. Besides, 3D printable green biocomposites are innovative and promising candidates for biodegradable and flexible materials as well as for sensors development for intelligent packaging applications. For instance, Dai et al. (2019) remarked that according to their rheological and mechanical properties, composite materials with cellulose nanofibres (CNF) are promising alternative for 3D printing. On the other hand, green biocomposites have lately been studied as expanded or foam-like materials used as loose-fill packaging and disposable food trays usually employed in food delivery in our modern urban society. Polystyrene (PS) is generally used for these applications, yet it is a petroleum-based and non-biodegradable polymer which is rather difficult to recollect and recycle in waste management systems, therefore new biodegradable and biobased alternatives are under investigation (Soykeabkaew et al. 2015). Davis and Song (2006) after analysing the biobased biodegradable packaging impact on waste management systems in comparison to

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oil-based polymer packaging materials, the first can have a positive environmental effect if composted in situ or provided an appropriate collection, transport, and special treatment of compostable waste. Diverse biocomposites and bionanocomposites have been developed and studied as sustainable alternatives for packaging materials due to the pressing environmental concerns derived from the extensive use of non-sustainable oil-based polymers. These materials require different properties depending on the product that must be protected within the package (i.e. food, pharmaceutical or electronic products), though good mechanical and gas barrier properties are most desirable characteristics.

4.1 Thermoplastic Green Biocomposites Thermoplastics green biocomposites present a wide range of properties and characteristics that are suitable for packaging applications, from blown-extrusion bags to thermoformed trays and containers. Most biodegradable materials present either poor mechanical or barrier properties, thus composite materials combining more than one type of bioplastic have been studied due to their enhanced final properties. One of the most widely used bioplastic nowadays is PLA since it is nontoxic, biodegradable, biocompatible, and presents comparable mechanical and thermal properties to conventional plastics such as PS and PET. Nevertheless, PLA has some existing limitations: such as inherent brittleness, low impact resistance, poor crystallization behaviour, poor toughness, etc. (Ghosh et al. 2019). In order to tailor the materials properties and reduce the production cost, blends of PLA with other bioplastics and biocomposites have been widely studied (Madhavan Nampoothiri et al. 2010). Burzic et al. (2019) found that PLA/PHA (80/20) blends significantly improved PLA crystallization and impact strength without comprising the biobased origin and biodegradable character of the materials. PLA biocomposites films by blown extrusion with functionalised chitosan and Arabic gum were developed by Ghosh et al. (2019), showing better tear resistance and thermomechanical properties and may as well stimulate the enzymatic biodegradation of the composites. Blends with starch have also been widely studied as a means of materials cost reduction and lessening of environmental impact, since starch is an abundant, renewable, biodegradable, and cheap biopolymer. Even lower environmental impact could be achieved by using starch derived from starch extraction wastewater as reported by Broeren et al. (2017). Due to the hygroscopic character of starch and the hydrophobic nature of PLA, weak adhesion is obtained in simple PLA/starch blends, being needed appropriate coupling strategies (Müller et al. 2016). Several research studies evaluated the grafting process and the use of compatibilizers with promising results (Lee and Wang 2006; Shirai et al. 2013; Yang et al. 2013; Akrami et al. 2016; Ibrahim and Ab Wahab 2017). De Oliveira et al. (2019) compared the life cycle impact of neat thermoplastic PLA and PLA/starch/cotton fibre biocomposites, the latter proved to have a better overall performance in terms of eco-efficiency though the end-of-life step of the material was not contemplated.

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Moreover, other PLA biocomposites with natural filler obtained from different waste materials, such as algae collected from beaches, trees sawdust or agro-industrial wastes, have shown a marked improve in films mechanical properties and good optical properties (Fortunati et al. 2015; Agustin-Salazar et al. 2018; Scaffaro et al. 2018; Orue et al. 2019; Silva et al. 2019). Generally, a surface treatment (i.e. alkali treatment) of the natural organic fillers is needed to improve adhesion between PLA matrix and filler particles. Likewise, a recent study on PLA composites with a natural inorganic filler showed that surface treatment drastically affect the biocomposites mechanical behaviour, observing a significant increase in materials toughness when calcium carbonate particles treated with stearic acid were used in presence of 20 %wt. of plasticiser (Aliotta et al. 2019). The authors concluded that the surface treatment is crucial for a better dispersion of filler particles in the matrix, thus when immersed in a plasticised polymer matrix (more ductile and with higher chain mobility) the dispersed filler dissipate energy by void formation, deboning and matrix ligament yielding. An interesting result was found by Singh et al. (2019) in PLA biocomposites reinforced with halloysite nanotubes and chitosan powder: the synergic effect of both fillers efficiently enhanced the materials ductility without compromising the tensile resistance even in the absence of plasticisers. Starch itself represents a promising cheap, fully biodegradable and a raw material with low environmental impact for green biocomposites. Starch blends with chitosan, gelatine or soy protein have also been reported for food packaging applications (Versino et al. 2016). Some starch-based blends are nowadays commercially available with trademarks as Mater-Bi® (Novamont, Italy), Bioplast® (Biotech, Germany), Novon™ (Chisso, Japan and Warner-Lambert, USA), Biopar® (Biop Biopolymer Technologies AG, Germany), Gaialene® (Roquette, France) and Solanyl® (Rodenburg Biopolymers, Netherlands) (Gurunathan et al. 2015). Besides, starch-based composite materials both with natural organic and inorganic fillers have been extensively studied and reviewed by Castillo et al. (2017). In previous works, bagasse and peels from root tubers starch extraction where used as biocomposites fillers based on corn and cassava TPS. An increase in the mechanical resistance of the reinforced materials up to 1.5% filler content was observed in films obtained by thermomoulding, though water resistance and permeability was not improved (Versino et al. 2015, 2019; López et al. 2015). Similar results were obtained by Merci et al. (2019) for cassava starch-based composites with soybean hulls or microcrystalline cellulose, showing lower water vapour permeability in composites with 2.5% of filler. Water vapour permeability and solubility of PLA/starch composite films were also improved by adding a chitosan crosslinked layer over the blend composite (Soares et al. 2013). Li et al. (2018) developed starch/nanocellulose biocomposites with good fibre dispersion up to 15% fibre content and improved mechanical resistance and hydrophobicity (increase in contact angle). Many nanocellulose based biocomposite have been developed and presented remarkable results in terms of optical, barrier, and mechanical properties; yet its production is considerably high energy consuming which lessens its eco-friendly character being necessary technological developments in this respect (Shanmugam et al. 2019). Consequently, nanocellulose based materials are being studied and developed for high-value applications,

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especially as biobased sustainable and innovative materials for flexible electronics, energy storage devices, and water treatment membranes (Fang et al. 2019). Starch as well as other biopolymers such as chitosan, carboxymethyl cellulose, and gelatine have poor mechanical performance and low water resistance, thus biocomposites and bionanocomposites are nowadays being studied as innovative ways of producing innovative novel sustainable packaging materials (Youssef and El-Sayed 2018). The development of the latter is of special interest in active or intelligent packaging (this will be discussed further on in this chapter), but its study and use have risen concern with respect to the nanoparticles’ diffusion and potential toxicity.

4.2 Green Foam Biocomposites Another type of materials that can be produced from green biocomposites is foam like materials, that have risen especial interest as an eco-friendlier alternative to expended polystyrene (PS) packaging. This non-biodegradable synthetic polymer is widely used in disposable food containers and packaging for products susceptible to mechanical damage. However it is difficult to collect in adequate conditions for its recycling. Besides, a life cycle assessment for warehouse appliances carried out by Razza et al. (2015) showed that starch based prototype packaging materials obtained by microwave technology, implied a reduction in non-renewable energy resources consumption (50%) and greenhouse gas emissions (60%) compared to expanded PS counterparts. In this regard, according to Guan and Hanna (2004), starch foams are a promising alternative for PS, especially for loose-fill packaging application. These materials should be light, compressible, and resistant enough to protect and stabilise packaged articles. Furthermore, starch can be easily processed by extrusion-cooking (method commonly used in the food industry) into loose-fill foam (Mitrus and Moscicki 2014). Starch-based foams can also be moulded into trays, another disposal packaging application in which they can substitute expanded PS materials. Starches from different botanical sources have been studied in this regard (Georges et al. 2018; Cruz-Tirado et al. 2019). Yet starch itself is a brittle and water sensitive polymer, thus the use of modified starches and its mixture with other bioplastics or fillers with better mechanical and water resistance have been widely reported. Soykeabkaew et al. (2015) summarised a wide range of processing techniques, types of starches modifications, and many blending examples used to overcome poor properties of native starch. In order to produce hydrophobic starch, acetylation and esterification have been studied by Bergel et al. (2018b). The authors found that contents up to 20%wt. of the modified starches reduced water absorption and increased impact resistance, though presenting higher density than the native starch foams. Similarly, Guan et al. had proposed acetylated starch composites foams both with corn cob fibre and cellulose (Guan and Hanna 2004) and with PLA (Guan et al. 2005). These authors evaluated processing conditions, additives type and quantity, and starch degree of acetylation on the final packaging properties. Pornsuksomboon et al. (2016) developed

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50/50 composite foams with native and citric acid modified cassava starch with higher thermal stability and lower water sorption, and similar thickness and density than native starch foams. Another recent work used crosslinked starch with cellulose biocomposites foams, with enhanced flexion properties and water resistance for 5%wt. cellulose contents (Hassan et al. 2019). Other authors have suggested the use of more hydrophobic and resistant bioplastics coatings as a means of overcoming starch foams detrimental characteristics with positive results (Bergel et al. 2017, 2018a). Starch composite foams blended with different concentrations of plant proteins, kraft fibre, and palm oil were studied by Kaisangsri et al. (2014) with the aim of improving the mechanical performance and water resistance of these biocomposites. All fillers reduced water absorption and water solubility index, especially zein and gluten protein, while kraft fibre gave the highest flexural and compressive strength, yet water resistance was significantly lower than the expanded polystyrene foam. The same authors developed starch/chitosan/kraft fibre biocomposites foams with lower density, higher water absorption, and higher mechanical resistance and flexibility than starch foams and starch/kraft fibre biocomposites (Kaisangsri et al. 2012). Likewise, Cruz-Tirado et al. (2017) used sugarcane bagasse and asparagus peel fibre as reinforcement, though no significant improvement was observed in mechanical properties up to 40% fibre loading. On the other hand, Mello and Mali (2014) have obtained trays from cassava starch and malt bagasse by thermo-moulding, and have thoroughly evaluated the effect of the presence of this agro-industrial by-product on final properties of the expanded trays, demonstrating that the addition of 10% of the residue significantly decreased their hygroscopicity. Similarly, the residue from pressing sesame cake has been incorporated into expanded cassava starch-based materials (Machado et al. 2017), showing that the incorporation of 20% or more of this residual product gave place to biocomposite foams with comparable mechanical properties to those of commercial expanded PS. Recently, different inorganic fillers were used as reinforcement of starch-based composite foams (Matsuda et al. 2013; Chiarathanakrit et al. 2019) as well as food waste mainly calcium based, were also revised as starch foams fillers (Chiarathanakrit et al. 2018; Kaewtatip et al. 2018). All biocomposites resulted in higher density, more resistant, and more thermally stable expanded materials. Remarkably, Kaewtatip et al. (2018) found that egg shell powder is a more effective filler than shrimp shells or commercial calcium carbonate. Natural fillers with other biodegradable bioplastics have been reported. In a recent work, Adu et al. (2019) developed an interesting highly strong and fully biodegradable PVA/nanocellulose biocomposite by an innovative freezing method, though the scaling and cost evaluation of this manufacturing system should be analysed. Moreover, Bocz et al. (2016) studied PLA biocomposites foams with cellulose and basalt fibres as fillers using supercritical CO2 as foaming agent, obtaining highly expanded materials with increased crystallinity than neat PLA foams. However, the best mechanical performance (compression test) was obtained when only talc was added as nucleating agent, attributed to the more homogenous cell formation. Furthermore, a biocomposite foam based on PBTA and peach palm tree fibres (an agricultural

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residue) was proposed by Pereira da Silva et al. (2017), showing enhanced mechanical properties with and without silane treatment of the fibres when compared to other natural fibres. However, better poymer/filler interaction and thus final material properties were obtained for treated filler. In summary, a wide range of biocomposite foams have been developed and studied over the last decade, being starch the most widely reported biopolymer used for packaging applications due to its comparatively lower cost. Nonetheless, in order to enhance these biobased and biodegradable composites, further investigations on the use of different cost-effective, low-energy-consuming, and innocuous foaming agents are necessary.

4.3 Active Green Composite Films The main objective of packaging is to retain the products goodness. Packaging design, quality, and efficiency are of great importance in food industry since it has to comply with several requirements, most often strictly regulated. Food packaging must try to extend the products shelf life as much as possible, preserving their organoleptic characteristics and nutrients, and guaranteeing food safety. It must protect the product form contamination at any point of the food supply chain (for example, cross contamination with other food products). Likewise, it must provide the consumer with the whole products composition and production information and be appealing from a marketing point of view. Consequently, the market demands a wide range of requirements and specifications for food packaging and labelling (Kalpana et al. 2019). Appropriate selection of packaging materials can help reduce food waste by helping consumers purchase the proper quantity they will consume and minimise food spoilage, while indirectly minimising energy and water consumption and plastic waste generation. According to the Food and Agriculture Organization (FAO, United Nations) estimation, almost half of the total worldwide food loses, that rise to approximately 1.3 billion tonnes per year, occur at processing, distribution, and consumption stages. Though intelligent and active packaging can help in terms of diminishing food waste, their use also increases packaging value and overall products cost, being needed in each case a cost-benefit analysis. Intelligent packaging has emerged as a means of information communication for producers, retailers, and consumers based on their ability to sense, detect, and record both external or internal changes in the product (Majid et al. 2018). Most indicators are intended for freshness and leakage detection by temperature or pH indicators, or by headspace gases and biological sensors integrated with data carriers such as bar codes and radio frequency identification (RFID) tags. An extensive and up-to-date review of intelligent packaging materials have been recently carried out by Kalpana et al. (2019), providing valuable information on different types of sensor and data carriers and combination on both biobased and conventional packaging materials that could be used or studied for tailoring intelligent packaging systems for specific applications.

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At large, intelligent packaging are a type of active packaging, meaning that a compound with a specific function is incorporated to the material, usually a polymer, which detects and also informs the consumer that some indicator of freshness or safety of the product has been altered during transport, distribution or storage. The use of active packaging in food is intended to extend the products preservation replacing conventional food processing techniques that are costly and can alter the food nutritional quality or produce undesirable organoleptic characteristics modifications (high thermal and pressure treatments, brining, acidification, dehydration and additive preservatives). Active materials are usually obtained by the development of composite materials including the active compound itself or other material particles that contain the active agent. These polymeric matrices have the potential of gradually releasing the active agents (antioxidants and antimicrobials) or absorbing undesirable food degradation products or promoters (ethylene, oxygen, and water) (Majid et al. 2018). One of the most studied biopolymer in active food films is chitosan due to its intrinsic antimicrobial activity. Chitosan is a biodegradable polymer derived from chitin, that can be extracted from oceanic food waste (shellfish processing byproducts) (Salaberria et al. 2015). The biocompatibility and antimicrobial activity of chitosan have promoted the development of innovative novel composites materials for wound dressings and drug controlled delivery systems (Jayakumar et al. 2011; Chen et al. 2013; Ghazaie et al. 2019; Zhang et al. 2019). Many biocomposite materials based on chitosan with antifungal and antimicrobial activity for packaging applications have been investigated (Lopez et al. 2014; Salaberria et al. 2015; Tan et al. 2015; Basu et al. 2017; Zhang et al. 2017; Niu et al. 2018; Moeini et al. 2019; Wang et al. 2019). Basu et al. (2017) developed chitosan based bionanocomposites reinforced with PLA nanoparticles loaded with quercetin (a flavonoid with bactericide and antioxidant capacity), with five times higher antioxidant capacity and microbial deterrent capacity, without compromising its water vapour barrier and mechanical performance. Likewise, a fully biobased and biodegradable composite from chitosan, ascorbate, and methylcellulose with antioxidant and UV-light absorption capacity were recently developed (Tan et al. 2020). An interesting composite material prepared with polyvinyl alcohol (PVA) and quaternary ammonium salt modified chitosan (HACC) was designed by Min et al. (2020) with both antifogging and antimicrobial activity. The antifogging characteristic is important in food packaging of fresh products for two clear reasons: fog hinders the products clear observation and water condensation on the package surface may lead as well to bacteria and fungi growth and further food deterioration. At present, the HACC/PVA coating was applied over PET trays to compare the shelf life extension of strawberries (as an example of fresh food preservation) but could equally be applied over bioplastic trays to produce a fully green biocomposite packaging. Another trend in active packaging materials for food industry involves the addition of essential natural oils as active compounds to biobased composite polymer matrices. Roy and Rhim (2019) developed gelatine/curcumin composite films with remarkable antimicrobial activity against foodborne pathogenic bacteria as well as good antioxidant activity. Another co-antimicrobial composite system based on chitosan

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and pine resin modified nanocellulose were proposed by Niu et al. (2018). Contrary to expectations, Kamdem et al. (2019) discovered that the inclusion of carvacrol to chitosan/xylan composites was not effective to increase the antimicrobial capacity of the films. A wide range of other natural essential oils based biocomposites were reviewed by Chiralt and Atar (2016). Furthermore, bionanocomposites have been investigated for intelligent and active packaging materials. Silver nanoparticles have proven to present antimicrobial activity being included in different bioplastic matrices (Ortega et al. 2017; Chi et al. 2019; Liang et al. 2019; Yu et al. 2019; Ceran et al. 2020). Passaretti et al. (2019) developed starch/bentonite (a natural clay) bionanocomposties with UVbarrier capacity as secondary package to protect coloured beverage or food products susceptible to UV radiation. A recent study presented unbleached softwood kraft sheets functionalised with gold nanoparticles as antioxidant packaging material, resulting in an integral green manufacturing process (Bumbudsanpharoke and Ko 2018). Antimicrobial biocomposites with zinc oxide nanoparticles were developed with different polymeric matrices: chitosan (Al-Naamani et al. 2016); gelatine (Ejaz et al. 2018); PVA/starch with pH sensing capacity (Jayakumar et al. 2019); PLA/PCL (Ahmed et al. 2019); biobased polyurethane and chitosan (Saral Sarojini et al. 2019). Many other innovative and interesting works have been recently reviewed by Youssef and El-Sayed (2018). However, since nanoparticles present a high diffusivity, their release during the use and end-of-life of the material and their impact on human health and the environment is currently being thoroughly studied (Han et al. 2018; Yu et al. 2019), yet further research on different nanoparticles and polymeric systems is needed.

4.4 3D Printing Composite Filaments Recently, 3D printing has arisen as an attractive technology for a vast range of new applications. Diverse materials from conventional thermoplastics and bioplastics to ceramics and metals are some of the materials that can be 3D printed. An interesting and complete review by Lee et al. (2017) summarises the different 3D printing technologies and materials developed to date, their operation fundaments, resolution and fabrication speed, energy consumption, properties, and thus its application feasibility. 3D printing offers the possibility of obtaining customised products in a relatively fast and low-cost manner, especially for applications were high precision reproducibility and design is needed, such as biomedical and electronics though nowadays is increasingly being used for massive products design for agriculture, healthcare, automotive, and aerospace industries (Sahrubudinh et al. 2019). Although, a large amount of research work on various 3D materials have been complied and reviewed in the literature lately (Lee et al. 2017; Bekas et al. 2019; Dai et al. 2019; Liu et al. 2019; Sahrubudinh et al. 2019), few studies have been reported on biocomposite materials for 3D printing. PLA biocomposite filaments with natural organic fibres have been developed with potential packaging applications

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(Murphy and Collins 2018; Le Duigou et al. 2019; Xiao et al. 2019). Cellulose based biocomposites have also been evaluated for potential 3D printing being a versatile polymeric matrix for bionanocomposites development for intelligent packaging (Dai et al. 2019). Likewise, an antioxidant PLA/lignin biocomposite was developed by Domínguez-Robles et al. (2019) for pharmaceutical and biomedical applications, though similar materials could be studied for active green packaging materials. 3D printing green bionanocomposites have a great potentiality to be employed in active and intelligent packaging, yet further investigations on biocomposites design and 3D printing technology and operation conditions optimisation are needed.

5 Environmental Impact Considerations Unfortunately, there is discrepancy with respect to whether “green biocomposites” are fully sustainable materials for two main reasons: 1. Their recyclability is limited due to thermal degradations in successive reprocessing cycles. 2. Their biodegradability is sometimes limited only to the filler (when polymer matrix is biobased but not biodegradable) and because composting processes are not always integrated to waste management systems. Therefore, the use of fully biobased and biodegradable materials and the improvement of waste management systems and regulations, together with the ecological consciousness education of producers, retailers, and consumers is crucial to move forward more sustainable packaging materials. Most recently, there is a growing tendency of consumers towards selecting eco-friendly and sustainable goods and, above all, those buyers more ecologically aware expressed that they were keen to pay more for these products (Han et al. 2009). This is key considering that bioplastics, and thus green biocomposites produced from them, still present high prices with respect to conventional polymers. Due to the growing bioplastics markets, prices are slowly dropping, yet further cost reductions are needed in order to have competitive prices (La Mantia and Morreale 2011). A more ecological and economical option is the use of residual biomass from current industrial processes as raw material for green biocomposites, with the aim of reducing costs as well as developing processes with minimum effluents generation towards circular production systems, especially in the case of biodegradable materials. Correspondingly, as previously stated, the use of biobased polymers and fillers presents various environmental advantages, mainly because of their biodegradable character, which helps reduce plastic waste generation and fossil-based resources use, minimising the carbon footprint. These aspects have been further investigated by life cycle assessment (LCA) of various biobased and biocomposite materials designed especially for packaging applications (Zabaniotou and Kassidi 2003; Bohlmann

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2004; Tencati et al. 2016; De Oliveira et al. 2019). The impact of agricultural production of most biobased compounds must be contemplated in LCA of green biocomposite materials, being eutrophication, fertiliser and pesticides use, and water and soil use and degradation the main environmental impact factors for this activity. Besides, the replacement of commodity polymers for biodegradable matrices or the inclusion of natural fillers usually require physical or chemical modification of the compounds in order to obtain similar or enhanced properties in comparison to conventional materials. These treatments imply additional production costs and can sometimes not be eco-friendly. Therefore, ideally future research should be conducted focusing not only on the mechanical, thermal, and barrier properties of the developed materials but also on their biodegradability under composting conditions. Besides, it is relevant the analysis of its overall effects on soil conditions; the toxicity and eco-compatibility of any used additive; the assessment of energy and resources (water, soil, etc.) consumption in both raw materials obtention and biocomposites processing; and the global life cycle analysis of the product considering the impact of raw material manufacture and real end-of-life of each produced material with the current waste management systems employed.

6 Conclusion and Future Perspective It is of common knowledge that every human activity has an impact on the environment. Global awareness has been lately raised on the consequence that the current socioeconomic system has brought. Consequently, sustainability is currently considered the guideline for present and future world development. The concept of sustainability implies a commitment of solidarity with future generation, achieving the desired socio-economic and cultural growth while making a rational use of the existing natural resources, preserving the species and the environment. In this regard, there is a worldwide trend towards the generation of regulations aimed at circularity of production systems. The objective of circular systems is to keep products, materials, and resources (water, energy) in circulation for as long as possible, minimising the generation of waste and reducing costs, hence achieving more sustainable processes. Packaging design and production play a key role in this respect, since packaging (a single use material) constitute an important source of solid waste. The development of biobased and biodegradable composite materials is needed for more environmentally sustainable packaging. Research towards developing materials with optimised properties in terms of product preservation, minimum resources use, and waste generation along the whole cradle-to-grave path will lead to more ecologically responsible packaging materials. This implies a reduction of energy, water, and soil use in raw materials manufacture, transport and distribution, storage and retail, consumption and disposal of both the product and the package. Biobased materials help transform primary products into value-added ones, particularly when residual biomass is used as raw material for biocomposites. This is an

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important growing opportunity for developing countries where agriculture is one of the main economic activities, playing a central role in the elaboration of strategies towards more equitable markets within the perspective of sustainable development. In addition, biomass derived compounds have a net lower environmental impact, though more ecological practices are being promoted as well in agricultural production to reduce solid waste generation, water eutrophication, and soil depletion problems. On the other hand, the biodegradability character of green biocomposites could help reduce plastic waste generation derived from the packaging industry if adequate composting systems are integrated to waste management programs. Additives and processes implied in their manufacture should also be as innocuous as possible in order to achieve totally green materials. Finally, the great versatility of biocomposites materials enables the development of customised packaging materials for different products as well as intelligent alternatives with enhanced properties, thus ensuring the quality, wholesomeness, integrity, and safety of the product for longer time. In summary, the development of green biocomposites for the packaging industry can meet the sustainability requirement of modern society.

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Towards Sustainable Buildings with Free-Form Geometries: Development and Application of Flexible NFRP in Load-Bearing Structures Hanaa Dahy

Abstract The application of non-renewable building materials including concrete and metals in construction industries have caused a major impact on the environment including the destruction of more than 45% of the global resources, the consumption of 35% of energy and nearly 40% of energy-related emissions (UN SecretaryGeneral’s High-Level Panel on Global 2012). In order to help in reducing these extending impacts, the building sector started considering three reduction of these dreadful impacts through increasing the environmentally-friendly building materials with Natural Fiber Reinforced Polymer Composites (NFRP), specially through WPCs (Wood Polymer Composites) since the 90s. The use of natural fibers in composites form in the building sector is historically applied since the phrase time when straw and mud were mixed to form the first known bricks in history. After the discovery of cement, it became the largest dominant material in the industry till our current time, in spite of its huge environmental damage. Natural fibres (NF) have been mostly applied in non-structural applications. Accordingly, this paper discusses applying reinforcement scenarios in the form of novel core and veneer reinforcement to enhance a load-bearing capacity to reach to improved mechanical properties that could enable building a demo- shell construction system. In this research natural fiber reinforced polymer composite (NFRP) produced from agricultural residues in the form of straw fibres (SF) mixed with three different types of polymers including (polylactide (PLA), a TPE (Thermoplastic elastic polymer) and high-density polyethylene (HDPE)) were developed through extrusion processes, then laminated or veneered to elevate the material properties needed to be reached to apply in load-bearing structures. The thermoplastic elastic polymers (TPE) were applied to enhance elasticity and flexibility in reaching sophisticated geometries. The H. Dahy (B) BioMat Department, University of Stuttgart, Institute of Building Structures and Structural Design, 70174 Stuttgart, Germany e-mail: [email protected] Department of Planning, Section of Sustainable Design and Transition, Aalborg University, 2450 Copenhagen SV, Denmark FEDA (Faculty of Engineering, Department of Architecture), Ain Shams University, 11517 Cairo, Egypt © Springer Nature Singapore Pte Ltd. 2021 M. T. Hameed Sultan et al. (eds.), Biocomposite Materials, Composites Science and Technology, https://doi.org/10.1007/978-981-33-4091-6_2

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mechanical strength was controlled by veneering. The targeted and reached material properties were applied in structural simulations that were later used in predicting the structural performance of a physical experimental shell construction that were built to validate the settled hypothesis of reinforcement of elastic/semi-elastic lignocellulosic cores to be applied in load-bearing systems. The paper will highlight the material samples development and testing, followed by an analysis and interpretation to the possibility of usage and appliance in load-bearing structures. Finally, the physical built demonstrator in the form of the experimental shell construction is shortly illustrated to showcase the validity of the settled hypothesis. Keywords Natural fiber reinforced polymer composite (NFRP) · Biocomposites · Sustainable construction system · Veneered biocomposites · Sustainable load-bearing systems

1 Introduction Biocomposite materials, i.e. natural fibre reinforced polymeric composites (NFRP) (Dahy 2017a, b) are materials fabricated from at least two main components, a fibre and a matrix (also known as binder), where at least one of the two main components is biomass-based. Wood consumption worldwide became questionable since the late twentieth century. This is due to the international trend of preserving the forests and retaining the needed eco-life equilibrium through changing the forestry behaviour and the amounts of consumed wood annually. Accordingly, searching for another available source of annually renewable natural fibres’ source became an international demand. On the other hand and since resource-intensive materials like steel, concrete etc. were and are still the predominant used materials in the building industry, different emerging technologies based on biocomposites’ application started growing since the 1990s, namely Wood Polymer Composites (WPC) (Vogt, C. Wood Plastic Composites (WPC) 2020) from wood and annually generated agricultural residues like straw bond by thermoplastics. Biocomposites using long lignocellulosic fibres started in the automotive industry by Ford in the 1940s, then faded for decades till it started back in the 1990s. Only few examples have been realized in the building industry. One of them is the pedestrian bridge made by Eindhoven University of Technology (Blok and Teuffel 2017). The main bridge girder was composed of hemp and flax fibres attached to a Polylactic Acid (PLA)-based foam core. Another example is the BioMat Research Pavilion completed in 2018 at University of Stuttgart Campus (Dahy 2019a, b) as well as the BioMat Tailored Fibre Placed (TFP) Biocomposite Camopy realized in 2019 (Dahy et al. 2020).

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Green biocomposites is a term that is used to describe that the main components of the biocomposite (fibre and binder) are both biomass-based. In that case, the positive value of applying those composites is doubled. One of the developed materials discussed here in the chapter is a green biocomposite that has been also veneered and mechanically tested.

2 Material Preparation Three NFRP materials were prepared using 70–80% straw fiber by wt. with a fiber size of 1–5 mm. The mixture was prepared by dry mixing the original components then directly extruding the composition using a double-screw lab-extruder without pre-pelletizing the biocomposite. Three types of polymers were used for production including polylactide (PLA), a thermoplastic elastic polymer (TPE) of PVA (Polyvinyl Acetate)-basis as well as high-density polyethylene (HDPE) polymer matrix, (Table 1). Before extrusion, the pre-blending of straw fibers and polymer matrix with coupling and lubricant agents were carried out. All biocomposite samples were extruded at an extrusion speed of average 18 cm/min, temperatures ranged between 120–160 °C and diverse pressing forces ranging from 30–140 Bar were applied to reach a final homogenous mixture with an acceptable extruded profile with no visinary trapped air gases in the surface. Mechanical tests were carried out on the three biocomposite charges that were visually and qualitatively pre-selected and inspected, showing the best quality out of the series of charges pre-compounded at different further fibre-ratios than those final selected ones given here. The mechanical properties of thee biocomposite samples were tested after oak veneer lamination was applied to the extruded samples for the aim of increasing bending strength and stiffness. Table 1 Biocomposites samples preparation

Charges

Material composition

Percentage (%)

1

Straw fibres (SF) Thermoplastic elastic polymer (TPE)

80 20

2

Straw fibres (SF) Polylactide (PLA) Thermoplastic elastic polymer (TPE) (Coupling agent)

70 15 13 2

3

Straw fibres (SF) High density polyethylene (HDPE) (Coupling agent)

70 28 2

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2.1 Material Lamination Process Through Veneering Lamination is a basic process known and applied in construction building materials, including the known laminated timber beam products. In this research, vacuum lamination processes were applied to enhance increased mechanical properties and strength through veneering the extruded NFRP samples. Before veneering through a selected PVA-based glue, the extruded biocomposite materials were rubbed to activate the capillary surface reaction to improve the gluing process. The veneering lamination process was assisted by means of vacuum appliance. The presence of a vacuum pump helped in eliminating air-pockets between the sandwich layers, composed of the extruded NFRP of the core and two oak veneer layers on both sides. The veneer itself was reinforced with plastic threads from the back-side, which helped in further reinforcement of the whole sandwich panel as needed. Applying vacuum has also assisted on reaching high-quality surfaces and the process took place for half an hour then left under pressure for a complete 24 h till the glue hardening process has been completed.

3 Standards for Test Specimens Preparation The test samples were prepared and calibrated according to the norm DIN EN 325:1993, the wood-based panels standard (DIN EN 1993). Samples were measured using digital caliper measurement with 0,01 accuracy. Due to extrusion irregularities, the samples’ thicknesses were measured from four points throughout the length and the thickness mean value was calculated accordingly, then typed as inputs in the used measuring machine. This is one of the necessary steps needed for testing prior to the bending test to be able to determine the elastic modulus. Further calibration conditions included preparing the samples at a relative humidity of (65 ± 5) % and a temperature of (23 ± 1) °C.

3.1 Bending Test Five samples were prepared from the pre-defined material cores that were presented in Table 1 and veneered with oak veneer from both sides to create the sandwich panels shown in (Fig. 1a) as described in 2.1 through vacuuming. The veneer fibres were fibre-oriented, which is different from the free-fibre orientation of the extruded core. A Universal bending machine (Zwick Z1455) setup for three-point bending test according to the pre-mentioned standard was applied hereby. The bending test setup included setting a 100 mm distance between both bottom supports. The upper supporting cylinder of the machine was 15 ± 0.5 mm and the loading cylinder diameter was also 30 ± 0.5 mm. The flexural loading speed was set to 25 mm/min

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Fig. 1 a Bending test specimens after lamination, b specimens under three-point bending test

with pre-load of 0.1 N/mm2 , according to the testing standard DIN EN 310:1993, Wood-based panels; for the determination of modulus of elasticity in bending and of bending strength (DIN EN 1993).

3.2 Tensile Test The tensile test was carried out on a universal testing machine set according to DIN EN 789 standard of tensile test for wood-based panels (DIN EN 2004). The testing machine setup under a uniform test speed of 10 mm/min with a pre-load of 0.01 N/mm2 at room temperature(23 ± 1 °C). The five tested samples had bone-like cross-sections according to the Fig. 2. The tensile test specimens dimensions: b2 = 42 mm, b1 = 28 mm, l3 = 210 mm, l2 = 171 mm, l1 = 90 mm and t = 6 mm.

Fig. 2 a Dimension of specimens for tensile test and b tensile test specimens after veneering

36 Table 2 Average elasticity modulus of biocomposite samples after bending and tensile test

H. Dahy Samples

No. of specimens

Elasticity modulus (GPa)

Tensile modulus (GPa)

1

5

1.8–2.5

–

2

5

4.5–5.2

1.1–1.5

3

5

5.5–6.3

1.4–2.8

4 Test Results and Discussion 4.1 Bending Test Table 2 shows the modulus of elasticity after the three-point bending test for five specimens of each material charge as defined in 3.1. Most of the samples were brittle prior to veneering, then the breaking performance was improved through the veneering reinforcement. For the first sample, the specimens has shown a good strength performance in compression stress from the applied bending force (Fig. 3a), exhibiting a clear elastic deformation due to the elastic properties already highly present in the TPE applied. In the second sample, a brittle like fracture were observed and weak flexural stress resistance existed. In the third sample, the straw fibers (SF) and HDPE composition were compatible, and the NFRP composite resulted here has shown the highest flexural stress resistance even before veneering took place. SF and HDPE were found the best biocomposite developed in this case with the best mechanical performance. Within the test and during breaking, delamination occurred, which was mostly at the bottom veneer layer due to the tensile stress created during loading condition.

4.2 Tensile Test During veneering, adhesives were applied and the methods used to activate the layers to which adhesion was applied, especially the core, have all played an important role in upgrading the NFRP biocomposites’ strengths. The value of tensile strength of the second veneered sample was around 1.1–1.5 GPa. In Fig. 4a, sample 2 from SF and PLA has shown high deformation. The average tensile strength of the third biocomposite sample from SF and HDPE ranged from 1.4–2.8 GPa after veneering was applied (Table 2) and its elasticity modulus increased 10 times after veneering was applied in comparison to previous measurements without veneering or reinforcement. It is worth mentioning that failures were mostly seen in the form of delamination and in the veneer layers themselves, which reflects that the biocomposites cores could not hold alone bearing tensile loads. This indicates the scope in this paper, that only through veneering to appropriate biocomposite core mixtures and with appropriate gluing conditions and glue types, advanced mechanical performances were reached

Towards Sustainable Buildings with Free-Form …

Fig. 3 Flexural stress versus deflection graph a sample 1, b sample 2 and c sample 3

Fig. 4 Tensile stress versus strain graph a sample 2 and b sample 3

37

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H. Dahy

and accordingly load-bearing elements were developed when correctly applied in certain appropriate loading conditions like that tested in the BioMat Pavilion 2018 in the form of a shell construction system as shown in part 5.

5 Potential Applications in Sustainable Building Structures In sustainable constructions, biocomposites can be used for small load-bearing structures such as pedestrian bridge structures (Blok and Teuffel 2017), cladding systems, flooring finishes and free-form furniture as from (Bioflexi) another development by the author (Dahy and Knippers 2016; Horn et al. 2018). The presented experiments were a part of the Research Project ‘BioProfile’ funded by Fachagentur Nachwachsende Rohstoffe e. V. (FNR) and conducted at the University of Stuttgart, by Department of Biobased Materials and Materials Cycles in Architecture (BioMat) at Institute of Building Structures and Structural Design (ITKE), lead by the BioMat director, the author. The BioProfile project pursues research on extruded and coextruded profiles for windows frames and exterior architectural applications, especially façade panels, made of plant residues as a source for natural fibres reinforced by biopolymers. The extruded profiles analyzed in this paper took place in the Fraunhofer- WKI center in Braunschweig in Germany with both the company ETS and BioMat assistances. During this research study, BioMat realized a Biocomposite Experimental Pavilion, which is an experimental load-bearing structure made using biocomposites. This pavilion that was realized in 2018 was a 3.6 m-high shell structure with a 9.5 m span, covering an area of 55 m2 . It was located on the campus of the University of Stuttgart in the city centre between August–December 2018, as shown in (Fig. 10). Its special feature is the modular construction of lightweight, single-curved elements that form a double-curved shell with individual segments consisting of veneer-reinforced elastic/semi-elastic biocomposite cores. The structure is supported by three crossed beams. The original target values for the settling of the shell global geometry was dependant on the results of the material developments given in this paper. The reinforcement strategy through veneering that enabled the original elastic/semi-elastic cores to reach higher mechanical performance were set as a main parameter in the whole development of the shell-construction. The initial structural simulations integrated those material properties, which have shown that the developed biocomposites would not allow a single shell layer, therefore a second structural layer was developed and attached to the first one. This interconnection and double layering enabled homogenous loads’ distribution in all directions. The presence of the three crossed laminated beams was essential and the loads were transferred to three superficial foundation footages that were located at different heights, according to the existing site conditions (Fig. 5). The parametric model of the shell was developed using Rhinoceros and Grasshopper softwares,

Towards Sustainable Buildings with Free-Form …

39

Fig. 5 Parametric model of the global geometry of the designed shell a perspective view on the 3D-laser scanned data and b layout of the model

enabling the control over the number of variations of the three-legged elements (Dahy et al. 2019a, b). In (Fig. 6), the fabrication of one element is illustrated and in (Fig. 7), the combination of the elements in one unit is illustrated. Consequently, 360 panels for the threelegged elements: upper elements, lower elements, upper edged elements and lower edged elements were fabricated (Fig. 8a) and the panels were assembled in 4 stages (Fig. 8b). In (Fig. 9) a detail of the relationship between the biocomposite panels and the wooden structural beam is illustrated and in (Fig. 10) the final experimental pavilion is shown.

Fig. 6 Lamination process a reinforcement of the bio-composites by 3D veneer layers and b vacuum-assisted veneer-reinforcement lamination process

40

H. Dahy

Fig. 7 Combining the legs to form the building units. a Combing each three leg-units and b combining six legs units to another three-legs unit to form the double shell structure

Fig. 8 Off-site and on-site construction phases a off-site segments ordering b sequence of fixing larger segments on site

Fig. 9 Detailing in the structure a relation between biocomposites and the timber beam b relation between diverse biocomposites units in the structure

6 Conclusion In this paper, an emphasis on veneering in elevanting the properties of biocomposite cores to reach up to load-bearing capacity for future sustainable constructions have been illustrated. The development of multi biocomposites extruded cores with freefibre oriented straw fibres was highlighted. It was intended to enduce the presence

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41

Fig. 10 Night-view for the biomat Pavilion 2018

of thermoplastic elastic polymers to enduce elasticity, hence flexibility in reaching sophisticated geometries and free-forms that are nowadays urgently needed in the building industry. On the other hand, it was intended to fix veneer though cloused vacuum-assisted lamination process that would fix the final geometries needed and enduce the targeted elevanted mechanical properties to build through the developed innovative building elements new structural applications. This was showcased through the built experimental pavilion in which the same target values for the developed materials was applied in the structural analysis. In the physical world, another biocomposite sandwich panel was developed using a different ligno-cellulosic elastic core, but the same mechanical values were applied. The modulus of elasticity was comparable to medium density fiberboard (MDF) and the target was reached to elevate the mechanical properties and to optimize the geometry of the shell to make proper advantage of the still relatively low mechanical properties in comparison to conventional glass fibre reinforced composites (GFRP). In this research, biocomposites were given a further chance to investigate elevating their properties to compete with conventional building materials towards more future sustainable constructions. Biocomposites have many advantages, but also disadvantages including water-absorption sensitivity. On the mechanical side, properties can be elevated by different means through oriented fibre-reinforcement as illustrated in this paper. But on the other hand, further research work is recommended to further go in that direction and especially to find environmentally-friendly means of improving the weathering resistance of these groups of materials. This will help enabling them to further compete globally in the building materials market and accordingly reach the needed target of reaching to a high percentage of appliance in reality, hence achieving a large percentage of targeting the urgently needed sustainability in this industrial field, the building industry world.

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H. Dahy

Acknowledgements The author thanks all students, employees, technicians and agencies involved in this project. This work was the direct result of cooperation between around 40 architecture students over two semesters participating in the (Flexible Forms) Design Studio in the semesters: WS17/18 and SS18 supervised by the author: Hanaa Dahy, the director of BioMat group at ITKE with her team: Jan Petrs, Piotr Baszynski and Michaela Mey, with the support from the University of Stuttgart and the Baden-Württemberg Foundation. Financial support was from the German Agency for Renewable Resources (FNR) under the Ministry of Food and Agriculture (BMEL), and scientific cooperation took place with Fraunhofer-Institut für Holzforschung (Fraunhofer WKI) and industrial partners Mathias Stange ETS and Profine GmbH in the framework of the Research Project (BioProfile) FKZ 22021516. International collaboration has taken place with the Technical University Eindhoven in the Netherlands (TU/e) - Department of Built Environment Prof. Patrick Teuffel, Dr. Arjan Habraken and M.Sc. Dirk Bos, in addition to local collaboration with various institutes of the University of Stuttgart, including the Institute of Construction and Structural Design (KE) - Prof. Urlike Kuhlmann and Mr. Janusch Töpler as well as the Institute of Engineering Geodesy (IIGS) Prof. Volker Schwieger and Gabriel Kerekes. Wooden beams were fabricated and supported by Burgbacher Holztechnologie GmbH with assistance of Mr. Steffen Haller, weathering resistant coatings were provided by Hexion Stuttgart GmbH.

References Blok R, Teuffel PM (25–28 September, 2017) Bio-based composite bridge—lessons learned published. In: Proceedings of the IASS annual symposium 2017 interfaces: architecture, engineering science. Hamburg, Germany Dahy H, Knippers J (2016) (PATENT) Flexible high-density fiberboard and method for manufacturing the same. EP3166765A1; Filed 8. July 2014 Published 13 January 2016. Based on Same Patent, Also Applied as WO2016005026A1; CN106604806A; US20170144327, EP2965882 A1 Dahy H (2017a) Biocomposite materials based on annual natural fibres and biopolymers—design, fabrication and customized applications in architecture. Constr Build Mater 147:212–220 Dahy H (2017b) Efficient fabrication of sustainable building products from annually generated nonwood cellulosic fibres and bioplastics with improved flammability resistance. Waste Biomass Valorizat No 1–9. https://doi.org/10.1007/s12649-017-0135-3. Springer Netherlands. Print ISSN 1877–2641, Online ISSN 1877-265X Dahy H (2019a) Materials as a design tool’ design philosophy applied in three innovative research pavilions out of sustainable building materials with controlled end-of-life scenarios. Buildings 9:64 Dahy H (2019b) Natural fibre-reinforced polymer composites (NFRP) fabricated from lignocellulosic fibres for future sustainable architectural applications, case studies: segmented-shell construction, acoustic panels, and furniture. Sensors 19(3):738 Dahy H, Petrs J, Bos DH, Baszynski P, Habraken APHW, Teuffel PM (2019a) BioMat Pavilion 2018: development, fabrication and erection of a double curved segmented shell from biocomposite elements. In: Lázaro C, Bletzinger KU, Oñate E (eds), Proceedings of the IASS annual symposium 2019—structural membranes 2019 form and force Dahy H, Petrs J, Baszynski P (2019b) Experimental biocomposite pavilion: segmented shell construction—design, material development and erection. ACADIA Conference 2019, Texas, USA Dahy H, Petrs J, Baszynski P (2020) Design and fabrication of two 1:1 architectural demonstrators based on biocomposites from annually renewable resources displaying a future vision for sustainable architecture. FABRICATE 2020, UCL, UK DIN EN 325:1993 (1993) Wood-based panels; determination of dimensions of test pieces. European Committee for Standardization

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DIN EN 310:1993 (1993) Wood-based panels; determination of modulus of elasticity in bending and of bending strength. European Committee for Standardization DIN EN 789:2004 (2004) Determination of mechanical properties of wood based panels. European Committee for Standardization Horn R, Dahy H, Gantner J, Speck O, Leistner P (2018) Bio-inspired sustainability assessment for building product development—concept and case study. Sustain Special Issue Sustain Construct 10(1):130. https://doi.org/10.3390/su10010130 UN Secretary-General’s High-Level Panel on Global Sustainability (2012) Resilient people, resilient planet: a future worth choosing, overview. United Nations: New York, NY, USA Vogt, C. Wood Plastic Composites (WPC) (27 January, 2020) Markets in North America, Japan and Europe with emphasis on Germany, 2005. Available online: http://biocompositescc.com/wpc data/File/06%20pr_wpcstudy_engl.pdf. Accessed on 27 Jan 2020

PLA Hybrid Composites Reinforced with Nanomaterials Narendra Reddy

Abstract Poly(lactic acid) (PLA) is the most common synthetic biopolymer that is derived from renewable resources and is biodegradable. PLA is used for commodity and specialized applications such as medical implants. Low processing temperatures, good mechanical and thermal stability and ability to obtain PLA in different configurations and structures make it an ideal biopolymer. However, PLA has several limitations, primarily, relatively poor resistance to chemicals. Several modifications have been done to improve the performance and properties of PLA and its products. Developing biocomposites by blending PLA with other biopolymers, nanomaterials including nanoclay have been done. Biocomposites have been developed by blending polysaccharides such as cellulose, starch, chitin and also proteins with PLA. Transparent and high performance biocomposites useful for food, medical and other applications have been developed from the blends. Despite numerous approaches, there are several limitations of PLA based biocomposites which presents scope for further research and development. Keywords Biocomposites · Blends · Polysaccharides · Proteins · Nanoclay · Nanowhiskers

1 Introduction Poly (lactic acid) is one of the most common synthetic biopolymers used extensively for food packaging, pharmaceutical, medical, textile and other applications. Biodegradability, production from a renewable resource, low melting temperature and solubility in common solvents are some of the advantages of using PLA. However, PLA is prone to hydrolysis under alkaline conditions, is relatively more susceptible to chemicals and products made from PLA do not have desired performance properties. Hence, considerable efforts have been made to modify PLA and improve its applications. Extensive studies have also been done to incorporate other N. Reddy (B) Center for Incubation Innovation Research and Consultancy, Jyothy Institute of Technology, Bengaluru, Thataguni Post 560109, India e-mail: [email protected] © Springer Nature Singapore Pte Ltd. 2021 M. T. Hameed Sultan et al. (eds.), Biocomposite Materials, Composites Science and Technology, https://doi.org/10.1007/978-981-33-4091-6_3

45

46

N. Reddy

biopolymers and additives to enhance the performance of PLA based materials. Nanomaterials (fibers, particles, whiskers, capsules etc.) have been extensively used to improve the performance of polymers. Similarly, nanoclays have also been known to increase the mechanical properties, thermal stability, compatibility etc. Extensive studies have been done to study the feasibility of improving the performance and applications of PLA materials through the addition of nanomaterials. In this chapter, we provide a review of the various cellulose, starch, protein and clay-based nanomaterials used to improve the properties, processing and applications of PLA.

2 PLA Reinforced with Cellulosic Nanomaterials Cellulosic materials have been the preferred reinforcement or additives for PLA to improve tensile, thermal, mechanical and performance properties. A variety of cellulosic materials including plants, trees, bacteria etc. are converted into nanofibrils, nanoparticles and nanowhiskers for potential reinforcement of PLA. Since cellulose is considerably stable and does not dissolve in common solvents, various approaches have been used to process cellulose and obtain the nanomaterials. Some of the common cellulosic materials used, techniques adopted for conversion and methods used to incorporate the nanomaterials into PLA are given in Table 1 (Kian et al. 2019). Films for food packaging applications were prepared using a blend of PLA, lignin nanoparticles, microcrystalline cellulose and umbelliferone (7-hydroxycoumarin), a natural plant extract with antioxidant and antimicrobial properties (Iglesias Montes et al. 2019). The three components were added in desired ratios into a twin screw extruder and processed into films at 180–200 °C. Thickness of the films formed was between 80 and 85 μm. Further, bilayer films were formed by assembling the monolayer films and compression molding at 155 °C for 1 min at a pressure of 25 bar. Interestingly, the thickness of the bilayer films was lower than the sum of the thicknesses of the individual layers. No major change in structure of PLA was observed due to the addition of the cellulose or lignin and morphological analysis showed good interaction between the components. Although films containing more than 15% antioxidant tended to be brittle due to increased inhomogeneity, highest elongation and strength were obtained in the films containing nanoparticles and the antioxidant (Table 2). Both lignin and umbelliferone contributed to the substantially high antioxidant properties of the films. Since hydrophobic PLA and hydrophilic microcrystalline cellulose (MCC) have poor interactions, chemical modification of MCC was done to improve the dispersion and compatibility with PLA. For the modification, MCC was acetylated using acetic acid to obtain an acetyl content of up to 17%. To obtain the nanocomposites, PLA was dissolved in chloroform to which the modified and unmodified MCC were added. Later, the mixture was poured onto Teflon molds to form nanocomposites with 0–to 17% of MCC. Acetylation helped in increasing the viscoelastic properties and compatibility. Addition of modified cellulose made

Toluene, ethanol; sodium chlorite, acetic acid; sodium bisulfite

Sodium hydroxide

Sodium hydroxide, sodium chlorite, acetic acid

Toluene, ethanol; sodium chlorite, acetic acid; sodium bisulphate; sodium hydroxide

Refined industrial flax stem fibers

Bleached flax yarns

Hemp fiber

Pristine hemp fiber

Sulfuric acid hydrolysis; physical modification with acid phosphate ester of ethoxylatednonylphenol surfactant

Defibrillation (microfluidizer)

Sulfuric acid hydrolysis

Sulfuric acid hydrolysis

Centrifugation, dialysis, sonication

–

Centrifugation, neutralization (by adding aqueous NaOH)

Centrifugation, ultrasonication, dialysis

Washing, centrifugation, dialysis, ultrasonication

Sodium hydroxide, sodium carbonate; hydrogen peroxide

Flax fabric

Sulfuric acid hydrolysis

Elemental chlorine-free (ECF) Hydrochloric acid hydrolysis Dialysis, sonication, vacuum bleaching sequence treatment filtration

Post-treatment

Flax fiber

Principal treatment

Chemical pretreatment

Source

(continued)

Evaporation casting of PLA/ PBS blend with functionalized nanocellulose in chloroform solvent

–

Evaporation casting of PLA in chloroform solvent

–

–

–

Fabrication techniques of PLA nanocomposites

Table 1 Methods used to convert cellulosic sources into nanomaterials and different approaches to reinforce them into PLA (Kian et al. 2019). Reproduced with permission from Elsevier

PLA Hybrid Composites Reinforced with Nanomaterials 47

–

Sodium hydroxide; sodium chlorite, sodium sulphite; hydrogen peroxide

Toluene, ethanol; soda cooking Homogenization (in basic reactor); hydrogen peroxide; hydrochloric acid hydrolysis (in ultrasonicator)

Sodium hydroxide; sodium hypochlorite, sodium sulphate

Hemp yarns

Jute fiber

Waste jute bags

Jute fiber

Sulfuric acid hydrolysis

Sulfuric acid hydrolysis

Sodium hydroxide, sodium hypochlorite (oxidation hydrolysis under continuous agitation and sonication)

Sulfuric acid hydrolysis

Toluene, ethanol; sodium chlorite, acetic acid; sodium bisulphate; sodium hydroxide

Hemp pristine fiber

Principal treatment

Chemical pretreatment

Source

Table 1 (continued)

Neutralization (by adding aqueous NaOH), discoloring (by adding aqueous NaOCl), vacuum filtration

–

Centrifugation

–

Centrifugation, dialysis, sonication

Post-treatment

–

–

–

–

–

(continued)

Fabrication techniques of PLA nanocomposites

48 N. Reddy

Chemical pretreatment

Caustic soda; sodium chlorite

Sodium hydroxide, dimethyl sulfoxide

Toluene; sodium hydroxide; hydrogen peroxide

Sodium hydroxide, dimethyl sulfoxide

Sodium hydroxide, dimethyl sulfoxide

Sodium hydroxide, anthraquinone (in rotatory digester); three-stage bleaching

Sodium hydroxide; sodium chlorite, acetic acid

Source

Jute fiber

Pristine jute fiber

Jute fiber

Jute fiber

Jute felts

Water retted kenaf fiber

Kenaf fiber

Table 1 (continued)

Centrifugation, mechanical agitation, sonication, neutralization (by adding aqueous NaOH)

Post-treatment

Filtration (using nylon membrane), neutralization (by rinsing with distilled water)

–

Sonication, neutralization (by adding aqueous NaOH), centrifugation

–

–

–

–

–

–

(continued)

Fabrication techniques of PLA nanocomposites

Sulfuric acid hydrolysis; Centrifugation, dialysis, – hydrochloric acid hydrolysis homogenization, addition with chloroform

Mechanical shearing (supermasscolloider); grinding

Sulfuric acid hydrolysis

TEMPO-mediated oxidation Centrifugation

Sulfuric acid hydrolysis; formic acid hydrolysis

TEMPO-mediated oxidation Centrifugation, high-speed homogenization, ultrasonication

Oxalic acid hydrolysis; steam explosion

Principal treatment

PLA Hybrid Composites Reinforced with Nanomaterials 49

Chemical pretreatment

Sodium hydroxide; sodium chlorite, acetic acid

Sodium hydroxide

–

–

Sodium hydroxide

Sodium hydroxide

Source

Kenaf core wood

Kenaf stem fiber

Roselle microcrystalline cellulose

Mulberry pulp

Ramie fiber

Ramie fiber

Table 1 (continued)

Sulfuric acid hydrolysis

Sulfuric acid hydrolysis; physical modification with organosilanization

Sulfuric acid hydrolysis

Sulfuric acid hydrolysis

Homogenization

Sulfuric acid hydrolysis

Principal treatment

Centrifugation, dialysis, homogenization, filtration, addition with chloroform

Centrifugation, dialysis, homogenization, filtration, neutralization (by adding aqueous NaOH)

Centrifugation, ultrasonication, dialysis

Centrifugation, dialysis, ultrasonication

–

Centrifugation, dialysis

Post-treatment

–

Melt-extrusion of PLA with functionalized nanocellulose in absence of solvent

–

–

–

–

Fabrication techniques of PLA nanocomposites

50 N. Reddy

PLA Hybrid Composites Reinforced with Nanomaterials Table 2 Tensile properties of monolayer and bilayer films of neat PLA and those reinforced with lignin, cellulose and umbelliferone (Iglesias Montes et al. 2019). Reproduced with permission through open access publishing

51

Sample

Modulus (GPa)

Strength (MPa)

Elongation (%)

Neat PLA

1.4 ± 0.2

41.5 ± 3.9

16.2 ± 4.2

PLA, 3% CNC 1.2 ± 0.1

28.1 ± 4.0

4.9 ± 0.8

PLA, 3% LNP

1.3 ± 0.1

34.2 ± 1.5

8.4 ± 0.9

PLA, 15% UMB

1.2 ± 0.3

28.8 ± 9.3

37.0 ± 4.2

PLA/PLA

1.6 ± 0.2

35.0 ± 6.2

16.4 ± 5.3

PLA/PLA, 3% CNC

1.4 ± 0.1

38.3 ± 2.9

5.6 ± 1.2

PLA/PLA, 3% LNP

1.5 ± 0.3

36.7 ± 4.1

6.3 ± 0.8

PLA/PLA, 15% UMB

1.6 ± 0.1

38.4 ± 2.2

4.5 ± 0.9

PLA 3CNC/PLA 15% UMB

2.1 ± 0.1

52.9 ± 2.2

2.9 ± 0.3

PLA 3LNP/PLA 15% UMB

2.0 ± 0.3

48.4 ± 4.4

3.0 ± 0.2

the films translucent (Fig. 1), reduced hydrophilicity and improved thermal stability. Acetylated cellulose had considerably higher dispersibility and also increased the Tg of PLA. Increase in modulus and decrease in strength was also observed with 17% cellulose in the composites (Tingaut et al. 2009). In another study, it has been shown that acetylated nanocellulose fibers (length 50 μm and diameter 50 nm) provided better phase affinity, higher dispersibility and orientation compared to nanocellulose coupled with silane or unmodified PLA (Ying et al. 2018). Coupling reaction did not affect the modulus of the nanofibers whereas acetylation decreased modulus by about 10% which was attributed to strong hydrogen bonding between PLA and acetylated nanocellulose. Morphological observations showed that pristine nanofibers had poor dispersion with aggregates formed of 1–2 μm in size. Both coupled and acetylated fibers had better dispersion based on visual observation (Fig. 2) and a parameter called interaction energy parameter. Acetylated nanofibers also provided increased tensile properties typically by about 38% but higher tensile strength up to 51% was possible depending on the amount of nanofibers in the blend (Fig. 3). Cellulose nanocrystals extracted from sugarcane bagasse were combined with PLA to develop composites with improved thermal stability (Gan and Wen 2019). Bagasse was delignified and later acid hydrolyzed with 6 M sulfuric acid and 11 M phosphoric acid at 55 °C for 30 min. Cellulose nanocrystals formed were obtained after dialysis and freeze drying. To form the composites, the nanocellulose and PLA were dissolved in chloroform to obtain a CNC concentration of 10%and the solution was cast to form films. Cellulose nanocrystals had diameters between 5.5 and 6.2 nm

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Fig. 1 Changes in the appearance (transparency) of neat PLA (a) films reinforced with acetylated microcrystalline cellulose. b (0%), c (3.5%), d (8.5%) and e (17%) nanocellulose reinforced films (Tingaut et al. 2009). Reproduced with permission from American Chemical Society

and length of 235–300 nm with aspect ratio between 37–55. However, agglomeration of the CNCs was also noticed due to intermolecular hydrogen bonding. XPS and FTIR studies confirmed the addition of sulphate and phosphate groups on the surface of cellulose due to the acid treatment. Composites containing phosphoric acid treated nanocellulose provided considerably higher thermal stability compared to neat or sulfuric acid treated cellulose (Gan and Wen 2019). In a related approach, highly crystalline cellulose nanofibers were modified to add a high density of carboxylate groups and later grafted onto PLA and made into composites. Cellulose pulp obtained from northern softwood and sugarcane bagasse pulp (non-wood) were treated with 2,2,6,6Tetramethylpiperidine-1-oxyl (TEMPO) and surface oxidation was done to obtain treated cellulose (Fig. 4). Ion-exchange with lithium chloride was done resulting in TEMPO oxidized cellulose nanofibers. The modified cellulose nanofibers were grafted onto PLA through ring open polymerization. Extent of grafting and molar ratios of carboxylate groups on the PLA were controlled by varying the reaction conditions. Carboxylate content of the materials increased significantly after treating with TEMPO and further after oxidation. A carboxylate content of 1.03 mmol/g was obtained for the oxidized cellulose compared to 0.03 mmol/g for the untreated sugarcane bagasse pulp. However, the crystallinity and crystal size of the modified cellulose was lower compared to unmodified cellulose pulp but higher than the crystallinity of neat PLA. It was suggested that inclusion of modified cellulose nanofibers would be a feasible approach to improve the thermal and performance properties of PLA for various applications (Chuensangjun et al. 2019a; b). An improved process and higher conversion of carboxylate groups along with higher crystallinity of cellulose

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Fig. 2 SEM (left) and TEM (right) images of neat (untreated fibers) PLA and PLA films containing coupled b and acetylated c nanocellulose fibers showing differences in dispersion and compatibility (Ying et al. 2018). Reproduced with permission from Springer Nature

was obtained when the ring open polymerization was done using lipase as catalyst. A carboxylate group content of 1.63–1.68 mmol/g was obtained and the PLA in the composites developed had crystallinity of 76% and crystal size of 3.8 nm. Better dispersibility of modified cellulose and higher thermal stability of composites was also possible by using the catalyst and ring open polymerization (Chuensangjun et al. 2019a; b). Grafting of cellulose nanocrystals onto PLA was also done using an in-situ ring opening polymerization approach of L-lactide. The cellulose nanocrystals were made from softwood pulp and had width of 13 nm and length of 140 nm. Nanocrystals and

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Fig. 3 Tensile strength and modulus of PLA films with untreated nanocellulose fibers (PLApNFs) and those containing nanocellulose fibers after coupling (PLAcNFs) and acetylation (PLAaNFs) (Ying et al. 2018). Reproduced with permission from Springer Nature

the L-lactide were dissolved in DMSO in the presence of a initiator and an catalyst. The polymerization reaction was performed over a period of 17 h under nitrogen atmosphere. Depending on the reaction conditions, a grafting ratio of 84–152% was obtained (Miao and Hamad 2016). The polymerized PLA containing nanocrystals was made into films using solution casting and also co-extrusion. Grafted nanomaterials had better dispersibility in chloroform compared to unmodified PLA. Considerable decrease in water contact angle but no major change in thermal behavior was observed due to the grafting of the nanocrystals. However, oxygen and water vapor permeability had decreased thereby increasing the barrier properties. Water vapor permeability rates between 2244 and 3137 (ml.mm)/(Cm2 .s.c.Hg) and oxygen transmission rates similar or better than that of commercially used PET, PE and cellulose acetate members were achieved making the PLA-celluose nanocrystal films suitable

PLA Hybrid Composites Reinforced with Nanomaterials

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Fig. 4 Schematic of the process of preparing the PLA composites grafted with cellulose nanofibers obtained from bagasse pulp modified using TEMPO (Chuensangjun et al. 2019a; b). The oxidation was done using lipase as catalyst to obtain higher carboxylate groups and crystallinity

for food packaging applications. Similarly, up to 1100% increase in modulus was possible since the nanocrystals functioned as excellent nucleating agent (Miao and Hamad 2016). An algae Posidonia oceanica has been used as a source of cellulose fibers and cellulose nanoparticles for reinforcing PLA. The algae was extensively bleached to obtain fibers and these fibers were converted into cellulose nanoparticles by acid hydrolysis with a yield of 14%. The nanoparticles were also modified with a surfactant to improve dispersion and compatibility when used as reinforcement in PLA nanocomposite films. PLA was dissolved in chloroform into which treated and untreated nanocellulose was added and the solution was made into 50 μm thick

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Fig. 5 Digital image a; AFM structure b, thermal behavior c and chemical structure d of the cellulose materials prepared from Posidonia oceanica (Fortunati et al. 2015). Reproduced with permission from Elsevier

films by solution casting (Fortunati et al. 2015). Nanocrystals obtained had a length of 180 nm and diameter of 4.9 nm and had typical cellulose I structure (Fig. 5). Acid hydrolysis reduced the thermal stability of the nanocellulose but did not affect the transparency of PLA films. Addition of the nanocrystals into PLA increased the porosity but reduced brittleness at concentration of 3% and after modification with the detergent. Also, better crystallization was observed in the PLA films containing the nanocellulose. Considerable increase in the mechanical properties of PLA particularly when reinforced with surfactant modified nanocellulose was observed (Table 3). Zinc oxide nanoparticles were used in addition to cellulose nanocrystals from Posidonia oceanica to further improve the properties of PLA for food packaging applications. Cellulose nanocrystals (length of 180 nm and diameter of 4.9 nm) extracted from Posidonia oceanica and zinc oxide nanoparticles were added into PLA (Luzi et al. 2017). The nanocrystals were modified by treating with phosphate ester of ethoxylatednonyl phenol and a surfactant. PLA and modified nanocellulose were dissolved in chloroform separately and later combined together (Fig. 6). ZnO nanoparticles were also dispersed in chloroform and combined with PLA to form binary blends and later combined with the cellulose and PLA solution to form tertiary

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Table 3 Effect of unmodified (CNC) and surfactant modified cellulose nanocrystals (SCNC) on mechanical properties of PLA films (Fortunati et al. 2015). Reproduced with permission from Elsevier Composition

Yield strength (MPa)

Neat PLA

Elongation at yield (%)

Breaking strength (MPa)

Breaking elongation (%)

Young’s modulus (MPa)

14.7 ± 5.1

2.3 ± 0.5

16.5 ± 3.1

278 ± 34

1205 ± 100

PLA, 1% CNC 31.3 ± 1.6

3.0 ± 0.5

27.1 ± 6.3

200 ± 53

1330 ± 90

22.9 ± 4.2

2.6 ± 0.6

22.9 ± 1.2

286 ± 25

993 ± 190

PLA, 3% CNC 42.1 ± 2.0

2.7 ± 0.6

21.9 ± 3.5

101 ± 28

1930 ± 170

25.7 ± 4.8

3.1 ± 0.1

20.2 ± 1.9

210 ± 48

800 ± 100

PLA, 1% SCNC PLA 3% SCNC

Fig. 6 Schematic of the preparation of cellulose nanocrystals for reinforcing with PLA (Luzi et al. 2017). Reproduced with permission through Creative Commons CC licence

blends (Luzi et al. 2017). Fibers were obtained after casting the solution and drying it under appropriate conditions. Addition of the nanomaterials into PLA separately did not affect the morphology of the nanocomposites whereas a more compact structure was formed in the case of the tertiary blends. Both tensile strength and elongation of PLA films increased due to the addition of the nanoparticles whereas water vapor permeability decreased which is preferable for food packaging applications (Table 4). There was no major effect of ZnO on the antimicrobial activity of the films. A combination of inorganic nanofillers and cellulose fibers were used to reinforce PLA and obtain composites with improved properties (Piekarska et al. 2016). PLA used had density of 1.24 g/cm3 with d and l-lactide content of 2.5 and 97.5%, respectively. Calcium carbonate (80 nm) (NCC), Closite 30B (MMT), and cellulose nanofibers (CF) were used individually and also in combination to reinforce

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Table 4 Mechanical properties and water vapor permeability of PLA reinforced with ZnO and/or cellulose nanocrystals (Luzi et al. 2017). Reproduced with permission through Creative Commons CC license Sample

Strength (MPa)

Elongation (%)

Modulus (MPa)

Water vapor permeability g mm/kPa h m2 11–53% RH

100–53% RH

PLA

16.5 ± 3.1

277 ± 33

1205 ± 100

0.051 ± 0.004

0.067 ± 0.006

PLA 1s-CNC

22.9 ± 1.2

286 ± 25

993 ± 190

0.047 ± 0.002

0.063 ± 0.002

PLA 0.1 ZnO

20.5 ± 2.0

105 ± 10

1163 ± 33

0.048 ± 0.005

0.063 ± 0.007

PLA 0.5 ZnO

38.3 ± 3.6

87 ± 8

1210 ± 50

0.046 ± 0.003

0.058 ± 0.002

PLA 1s-CNC 0.1 ZnO

21.7 ± 1.6

250 ± 45

975 ± 60

0.044 ± 0.001

0.056 ± 0.001

PLA 1s-CNC 0.5 ZnO

23.7 ± 0.8

205 ± 15

990 ± 40

0.041 ± 0.002

0.052 ± 0.005

the PLA. PLA blends with NCC and MMT were made by melt mixing the materials and later combining with CF, when hybrid films were developed. Films of thickness 1 and 0.5 mm were extruded which were later compression molded into designated samples at 180 °C for 3 min. The clay particles had good dispersibility and did not show any agglomeration and addition of CF favored further dispersion. However, thermal stability of the hybrid PLA/MMT/CF films was lower than that of PLA/MMT but better than that of PLA/CF. Tensile properties of PLA improved with addition of both MMT or NCC but the improvement was further enhanced when CF was included into the MMT and NCC (Table 5). PLLA based surfactants were developed and used as matrix with cellulose nanocrystals extracted from ramie fibers (Mariano et al. 2017). Nanocellulose obtained had average length of 200 nm, width of 7 nm and % crystallinity was 85%. The PLA based surfactants (PEG-b-PLLA) and Im-PLLA were prepared from L-lactide and other chemicals through ring opening polymerization. The prepared surfactants were combined with CNCs and solution cast to form films on a glass plate. Films formed were combined with PLA and extruded in a twin screw extruder at 170 °C and 200 bar pressure to form the composite samples. Addition of the surfactant PEG-b-PLLA increased the transmittance whereas Im-PLLA decreased the transmittance as indicated in Fig. 7. Thermal stability of the films had also increased due to the addition of Im-PLLA which was suggested to be due to the improved ionic interactions between the polymer and cellulose nanocrystals. However, it was also postulated that the surfactant could cause a decrease in molecular weight and shortening of PLA chains which leads to plasticization effect and reduces the crystallinity of the matrix (Mariano et al. 2017).

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Table 5 The properties of PLA films reinforced with MMT, NCC, CF and their hybrids (Piekarska et al. 2016). Reproduced with permission from Elsevier Sample

Temperature (°C)

Yield stress (MPa)

Stress at break Elongation (MPa) (%)

Storage modulus (MPa)

PLA

25

49.3 ± 0.1

47.3 ± 0.1

29.0 ± 3.8

3011 ± 43

PLA/CF

25

44.8 ± 1.2

7.1 ± 0.3

3500 ± 8

PLA/NCC

25

42.2 ± 2.2

31.7 ± 0.7

3296 ± 4

PLA/NCC/CF

25

PLA/MMT

25

PLA/MMT/CF

25

– 44.1 ± 2.4 – 49.9 ± 1.6 –

41.5 ± 0.1

5.8 ± 0.5

4340 ± 45

45.1 ± 0.7

7.8 ± 0.6

3600 ± 63

42.1 ± 2.1

5.1 ± 0.3

4578 ± 34 2938 ± 43

PLA

35

47.5 ± 0.5

42.8 ± 0.7

37.4 ± 6.4

PLA/CF

35

33.4 ± 3.0

32.5 ± 1.4

6.2 ± 0.1

3410 ± 8

PLA/NCC

35

43.0 ± 0.7

32.7 ± 1.8

47.4 ± 4.0

3216 ± 2

PLA/NCC/CF

35

35.8 ± 2.9

29.8 ± 1.0

6.2 ± 0.1

4224 ± 46

PLA/MMT

35

43.8 ± 0.6

37.2 ± 0.2

9.4 ± 1.3

3515 ± 47

PLA/MMT/CF

35

28.7 ± 4.0

27.6 ± 4.1

4.4 ± 0.3

4444 ± 58

Fig. 7 Effect of adding various levels of modified surfactants onto PLA and injection molded into discs a and image of unmodified PLA composites (Mariano et al. 2017). Reproduced with permission from American Chemical Society through open access publishing

A ternary blend of microcrystalline cellulose, nanocrystal cellulose and nanolignin was combined with unmodified and modified PLA to develop films with antimicrobial properties for food packaging applications (Yang et al. 2016). Different combinations of the three polymers were used to develop films with varying properties. Films containing 3% nanocellulose had good transparency to UV light but the presence of lignin in the films led to blocking of UV light. Changes were also observed in the thermal and crystal behavior which led to grafted PLA providing better compatibility and hence improved crystallization and thermal resistance. A lowest crystallization temperature of 96 °C was possible due to the presence of cellulose and lignin nanoparticles. Addition of cellulose led to larger increase in tensile properties compared to lignin nanoparticles. Pure PLA films were susceptible to microbes whereas addition of 3% lignin provided high resistance and even 1% cellulose in the ternary blend

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Fig. 8 Ability of PLA and cellulose nanoparticles and lignin nanoparticle containing blend films to resist growth of Pseudomonas syringae pv. tomato (Pst) after incubation for up to 24 h (Yang et al. 2016). Reproduced with permission from Elsevier

leads to marginal decrease (Fig. 8). It was suggested that the ternary films could be used to detect pathogens in food when used as packaging (Yang et al. 2016). Nanowhiskers extracted from oil palm empty fruit bunch fibers were used to improve the properties of PLA films (Haafiz et al. 2016). Cellulose nanowhiskers (with lengths of about 100 nm and widths of about 10 nm) were extracted using the acid hydrolysis approach. Chemical modifications were done to the whiskers to improve swelling. Modified nanocellulose whiskers were added into PLA dissolved in chloroform in 1, 3 or 5 weight % and cast into films. Atomic force microscopy (AFM) images confirmed the nanoscale dimensions but revealed that the cellulose was in spherical shape before hydrolysis but converted into rod like whiskers after hydrolysis. Uniform distribution and good interaction but poor interfacial adhesion was observed between nanowhiskers and PLA up to a fiber content of 3%. Higher levels of reinforcement led to agglomeration and also reduction in properties. Due to these changes, strength and modulus of the films increases but elongation decreases with increasing levels of nanocellulose in the films. However, thermal behavior of PLA films was found to improve after the addition of the nanocellulose (Haafiz et al. 2016).

3 PLA Reinforced with Nanofibers from Bacteria Similar to plant sources, nanofibrils obtained from bacterial cellulose have also been studied for potential improvement in the performance of PLA films (Martinez-Sanz et al. 2012). Bacterial cellulose was prepared from the strain Gluconacetobacter xylinus 7351 and used to improve the properties of PLA having molecular weight of 150,000 g/mol. The cellulose developed was treated with sulfuric acid at 50 °C for three days to form the nanowhiskers with a yield of about 79%. The PLA and bacterial cellulose nanowhiskers were dissolved using HFIP and electrospun into fibers using a voltage of 10–12 kV and extrusion speed of 0.66 mL/h. Nanocomposites were

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developed from the cellulose using three different approaches: electrospinning, solution casting and injection molding. Films developed using electrospun nanowhiskers showed lesser reduction in transparency and more homogenous distribution. Considerable differences in fracture morphology were observed due to the differences in compatibility (Fig. 9). Melting and glass transition were not affected due to the addition of the nanocellulose. However, melting enthalpy and cold crystallization

Fig. 9 SEM images of pure PLA a PLA films made using 16% EVOH b, 1% bacterial cellulose containing film made using electrospinning c, 2% bacterial cellulose containing film made using electrospinning d, 2% cellulose freeze dried e, 2% bacterial cellulose made using extrusion, f (Martínez-Sanz et al. 2012). Reproduced with permission from American Chemical Society

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Table 6 Thermal behavior of PLA and PLA nanocomposites containing bacterial cellulose incorporated into PLA using various techniques. Melting enthalpy (Hm), cold crystallization temperature (Tcc), cold crystallization enthalpy (HCC), PLA crystallinity (Xc), and glass transition temperature (Tg) (Martínez-Sanz et al. 2012). Reproduced with permission from American Chemical Society Samples Tm (°C)

Hm (J/g) TCC (°C) PLA

HCC (J/g) Xc (%) PLA

Tg1 (°C)

Tg2 (°C)

PLA

150.8 ± 2.5 26 ± 0.7

119 ± 1.8

12 ± 0.5

16 ± 0.3 60 ± 1.8 60 ± 1.1

1% ES

151.6 ± 1.5 22 ± 0.5

105 ± 0.9 4.3 ± 3.7

19 ± 4.6 59 ± 0.0 59 ± 0.3

2% ES

148.5 ± 2.0 25 ± 1.1

110 ± 1.4

13 ± 0.9

13 ± 0.2 58 ± 0.5 59 ± 0.8

3% ES

150.3 ± 3.2 18 ± 0.6

109 ± 1.9 2.6 ± 1.6

17 ± 1.1 59 ± 0.6 59 ± 0.3

1% FD

150.9 ± 2.1 24 ± 0.8

119 ± 1.6 9.4 ± 0.6

16 ± 1.1 58 ± 1.3 59 ± 1.0

2% FD

150.2 ± 0.7 24 ± 2.9

120 ± 3.3 9.3 ± 0.4

16 ± 2.6 57 ± 2.3 59 ± 0.1

PLA, 16% EVOH

151.3 ± 1.5 22 ± 1.0

123 ± 1.7 8.1 ± 1.9

15 ± 1.0 56 ± 0.5 59 ± 0.5

1% P

151.0 ± 2.4 27 ± 0.4

121 ± 2.1

11 ± 0.4

18 ± 0.8 59 ± 1.3 59 ± 1.3

2% P

149.8 ± 0.1 26 ± 0.1

120 ± 1.2 7.6 ± 3.7

19 ± 3.9 59 ± 1.2 59 ± 0.3

temperature were affected by the method adopted to add the cellulose nanowhiskers into PLA (Table 6). About 17% increase in elastic modulus and 14% in tensile strength were obtained when 2–3% nanowhiskers were incorporated. Water vapor permeability decreased by 43% at 3% fiber loading whereas significant improvements in oxygen barrier properties were noticed even when the nanofiber loading was 1%. Bacterial nanocellulose, particularly after electrospinning were found ideal to improve the barrier and mechanical properties required for food packaging and other applications (MartínezSanz et al. 2012). Bacterial cellulose pellicles prepared by mechanical disintegration having width of about 150 nm and tape-like structure were added into molten PLA (molecular weight between 195,000 and 205,00 Da and density of 1.24 g/cm3 ) and extruded in the form of sheets of about 0.5 mm thickness. No changes were observed in the degradation temperature of PLA but the glass transition and cold crystallization was found to increase due to the addition of bacterial cellulose. Ability of the bacterial cellulose to act as a nucleating agent and even dispersion in the matrix were suggested to be the reasons for the changes in thermal behavior. Good interfacial interaction leading to decreased segmental mobility lead to increase in modulus. About 15% increase in strength and 10% increase in tensile modulus was possible even with an addition of 5% of nanofibers (Panaitescu et.al. 2017). It was observed that compression molding sheets of electrospun PLA nanofibers at specific temperature and time leads to formation of transparent films with substantial improvement in toughness (Somord et al. 2018). Utilizing this phenomenon of selfreinforcement, PLA nanocomposites with improved performance properties were prepared by adding modified and unmodified bacterial cellulose (Somord et al. 2018). Bacterial cellulose was cultivated from Gluconacetobacter xylinus strain TISTR 975

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and obtained in the form of pellicles with thickness of 1 cm. These pellicles were homogenized and maintained as a suspension in water. The bacterial cellulose was also treated with 3-(trimethyoxysilyl) propyl methacrylate to improve adhesion and compatibility with PLA. Electrospun PLA mats were placed in a filter and BC or modified BC (MBC) suspension (1 or 10%) was poured onto the mats. Additional PLA mats were stacked and BC was added onto the PLA as shown in Fig. 10. After the desired layers of PLA and BC were added, the multilayer structure was treated with ethanol, rinsed in water and dried at 50 °C for 150 min. Rectangular pieces of 3 cm × 3 cm × 150 μm were cut from the PLA-BC treated membranes and placed in a mold and compression molded to form films. Films without and those containing 1% cellulose were transparent but the transparency decreased with increasing bacterial cellulose content. Addition of BC lead to improvement in tensile properties due to the reinforcing effect which was more pronounced when MBC was used (Table 7). Thermal resistance increased and water vapor permeability had decreased after the addition of bacterial cellulose (Somord et al. 2018).

Fig. 10 Schematic of the process of converting bacterial cellulose reinforced PLA nanocomposites through self-reinforcement technique (Somord et al. 2018). Reproduced with permission through Creative Commons Attribution License

Table 7 Tensile properties of neat PLA electrospun membranes, self-reinforced PLA and those reinforced with 1 or 10% of BC or MBC (Somord et al. 2018). Reproduced with permission through Creative Commons Attribution License Sample

Strength (MPa) Modulus (GPa) Elongation (%) Toughness (MJ/m3 )

PLA

49.9 ± 3.0

2.8 ± 0.1

5.2 ± 2.4

2.2 ± 1.0 8.5 ± 5.0

Neat SR-PLA

61.2 ± 2.0

2.2 ± 0.3

15.6 ± 8.9

SR-PLA-1% BC

70.1 ± 2.0

3.0 ± 0.2

0.2 ± 1.2

3.6 ± 0.9

SR-PLA-1% MBC

79.5 ± 4.9

3.1 ± 0.3

46.8 ± 6.2

35.0 ± 4.4

SR-PLA-10% BC

79.6 ± 1.8

3.5 ± 0.1

3.1 ± 0.3

1.7 ± 0.3

SR-PLA-10% MBC 87.5 ± 3.5

3.8 ± 0.1

2.8 ± 0.2

1.9 ± 0.5

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Fig. 11 Tensile properites of PLA films reinforced with cellulose nanofibrils (CNC) and bacterial nanocellulose (BNC) with increasing content of nanofibers (Gitari et al. 2019)

In a comparative study, it was found that bacterial nanocellulose was able to provide better mechanical properties than cellulose nanofibrils obtained from softwood (Gitari et al. 2019). It was reported that during the extraction of nanofibers from bacteria, formation of carbonate occurs which reduces bonding between the nanocellulose and improves dispersion in the PLA matrix. Both the nanofibers and nanocellulose decreased glass transition whereas nanofibrils increased and nanocellulose decreased the cold crystallization temperature. In terms of mechanical properties, there was an increase of Young’s modulus from about 2500 to 3672 MPa. Similarly, strength of the composites containing bacterial nanocellulose increased to 50 MPa, higher than that for the cellulose fibers (Fig. 11). Elongation and toughness of the films decreased due to the addition of the nanomaterials (Gitari et al. 2019).

4 PLA Composites Reinforced with Chitin-Based Nanomaterials In addition to cellulose, other polysaccharides particularly starch and chitin/chitosan have also been used as reinforcement for developing PLA nanocomposites. A summary of the various attempts made to use chitin and starch in PLA nanocomposites is given in Table 8. As seen from the table, both nanocrystals and nanofibrils from chitin have been formed and incorporated into PLA through solution casting or thermoplastic processing. Substantial improvements in mechanical and performance properties could be achieved due to the reinforcing effect (Scaffaro et al. 2017). In a recent study by Coltelli et al., chitin nanofibrils (20 μm in length and 90 nm in width) were combined with poly(ethylene glycol) (PEG) and used to develop PLA nanocomposites. Since chitin nanofibrils tend to agglomerate in water suspensions and it is difficult to incorporate them into PLA, the fibrils were first combined with PEG in 1:1 ratio and made into pre-composites. This pre-composite was combined with PLA

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Table 8 Types of chitin/starch used as reinforcement for developing PLA nanocomposites and resulting properties (Scaffaro et al. 2017). Reproduced with permission from Springer Nature Content (wt%)

Filler modification

Other components

Preparation method

Main results

ChNCs up to 10%

Acetylation

–

Solvent casting

E = +37%, TS = +45%

ChNCs 1% –

TEC

Extrusion

E = +67%, TS = +28%

ChNCs 1% –

Glycerol triacetate, Film blowing polybutylene adipate-co-terephthalate, talc

E = +75%, TS = +65%

ChNCs up to 5%

–

PHBV, DMAC

CO2 Foaming

TS = +133%

ChNCs up to 10%

l-Lactide

–

Solvent casting

E = +133%, TS = +63%

ChNCs up to 10%

l-Lactide

–

Electrospinning

E = +143%, TS = +49%

ChNCs up to 5%

l-Lactide

MgO

Injection molding

E = + 18%, TS = +33%

SNCs 3.2%

PLA

–

Grafting reaction

Successful grafting of PLA onto SNCs

SNCs up to 8%

Cross-linking

–

Solvent casting

E = +35%, TS = +12%

SNCs up to 30%

PCL

–

Solvent casting

TS = +132%, εb = +400%

SNCs up to 2%

Ozone-mediated styrene grafting

–

Solvent casting

WVP up to − 52%, DTGpeak up to +130 °C

in a twin screw extruder and made into specimens by injection molding at 180 °C, 650 bars pressure, holding time of 15 s and mold temperature of 135 °C. Addition of PEG increased the compatibility between chitin and PLA leading to improved thermal resistance. Although no major improvements in tensile strength or modulus were observed, elongation of the nanocomposites was high at about 150%. It was suggested that PEG formed a coating on the nanofibrils and improved extensibility but formed a layer between chitin and PLA preventing reinforcing effect (Coltelli et al. 2019). In another approach, chitin obtained from crabs were made into nanowhiskers by acid hydrolysis and later by treating chitin powder in 5% KOH solution and boiling for 6 h. The powder obtained was bleached and retreated with 3 mol/L HCl at 102 °C for 5 h. The obtained chitin nanowhiskers were further modified by grafting l-lactide via ring-opening polymerization. The modified and unmodified chitin whiskers were added into PLA dissolved in chloroform and later solution cast to form films. FTIR and NMR studies confirmed successful grafting of oligo(L-lactide) side chains onto

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Fig. 12 SEM (a and b) and TEM (c and d) images of unmodified and grafted chitin nanowhiskers (Li et al. 2016). Reproduced with permission from Elsevier

chitin. No major changes in the crystallinity or crystal size of chitin were noticed due to the grafting. Length of unmodified nanowhiskers was between 200 and 800 nm and diameter was between 20 and 70 nm (Li et al. 2016). Comparatively, the grafted whiskers had length between 150 and 400 nm and diameter was between 5 and 55 nm. The decrease in the diameter of the fibrils and better dispersibility in PLA matrix was noticed in the SEM and TEM images (Fig. 12) (Li et al. 2016). When incorporated into PLA, the whiskers caused changes in the crystallization rates and spherulite formation. Marginal decrease in crystallinity and melting temperature was also noticed. Substantial increase in tensile properties with highest strength of 30.5 MPa and modulus of 1.5 GPa and fracture energy of 333 J/mm were obtained when 5% grafted whiskers were used as reinforcement (Fig. 13). Nanocomposites containing grafted PLA also exhibited better cytocompatibility to MC3T3-E1 cells. Considerable effect of chitin nanocrystals (diameter between 7 and 25 nm, length between 314 and 1571 nm and crystallinity of 91%) on the crystallinity, thermal behavior and mechanical properties was observed when used as reinforcement for triethyl citrate plasticized poly (lactic acid). Chitin nanoparticles and plasticizer were dissolved and made into a solution and added into an extruder along with PLA. Amount of components added was controlled to obtain a nanocrystal concentration of 1%. PLA pellets extruded were compression molded into films by processing at 190 °C for 3.5 min at a pressure of 4 MPa. After compression, the films were cooled to room temperatures at two different rates (fast

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Fig. 13 Effect of addition of unmodified and grafted chitin nanowhiskers on the tensile properties of PLA nanocomposites (Li et al. 2016). Reproduced with permission from Elsevier

cooling for 5 min or slow cooling for 20 min) to vary the crystallization rate and extent of PLA. Differences in the cooling rates affected the crystallinity, transparency and tensile properties, particularly elongation. Faster cooling provided films with higher transparency (Fig. 15) and crystallinity and improvement in tensile properties was observed until 1% reinforcement (Hererra et al. 2016). Changes in the tensile properties of the PLA nanocomposites obtained using fast or slow cooling are shown in Table 9 and Fig. 14.

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Table 9 Effect of fast cooling (FC) and slow cooling (SC) on the tensile properties of PLA films with and without chitin nanoparticles (Hererra et al. 2016). Reproduced with permission from Elsevier Type of PLA

FC PLA-TEC FC-PLA-TEC-ChNc

Yield strength (MPa)

Ultimate strength (MPa)

Young’s modulus (GPa)

Elongation at break (%)

Work of fracture (MJ/m2 )

3.7 ± 1.2

22.4 ± 4.1

0.3 ± 0.2

305 ± 53

32.2 ± 12.8

21.4 ± 3.9

21.0 ± 2.0

1.2 ± 0.4

262 ± 24

36.9 ± 4.4

SC PLA-TEC

19.6 ± 1.2

15.8 ± 1.1

0.6 ± 0.1

91 ± 38

16.1 ± 6.4

SC-PLA-TEC-ChNc

20.9 ± 0.5

20.3 ± 0.5

1.0 ± 0.3

16 ± 4

2.9 ± 0.8

Fig. 14 Influence of cooling rates on the transparency of neat and chitin nanowhisker reinforced PLA as seen visually and using an optical microscope (Hererra et al. 2016). Reproduced with permission from Elsevier

5 PLA Containing Starch-Based Nanomaterials Starch nanocrystals prepared from two sources, high amylopectin maize starch and acetylated starch were prepared and used as fillers for PLA. Starch nanocrystals were prepared by acid hydrolysis and later acetylated by treating with glacial acetic acid to obtain a degree of substitution of 0.99. The PLA and starch nanoparticles were dispersed in dichloromethane and cast onto glass plates to form films. PLA reinforced with unmodified starch had a rougher morphology due to agglomeration of the particles and poor bonding between PLA and starch. The roughness was found to increase with increasing amount of nanofillers. Acetylation of starch improved adhesion and interfacial interaction that provided a relatively smoother surface to the nanocomposites (Fig. 15). A significant decrease (25%) in oxygen permeability was

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Fig. 15 TEM images of PLA films containing unmodified (left) and acetylated starch nanocrystals (right) show differences in agglomeration and interaction (Takkalkar et al. 2019). Reproduced with permission from Springer Nature

observed for the films reinforced with acetylated nanostarch. Similarly, the mechanical properties of the starch nanofiller containing PLA were higher but elongation was lower compared to neat PLA (Takkalkar et al. 2019). Starch nanocrystals were prepared by acid hydrolysis of starch by treating with sulfuric acid at 40 °C for 5 days. Lactic acid was grafted onto the nanocrystals using tetrahydrofuran at 60 °C for 30 min. PLA nanocomposites were prepared by adding the modified starch (5–30%) into PLA solution and later casting the solution into films (Zamir et al. 2019). Grafted samples showed a decrease in atomic concentration of C-H from 13 to 4% and an increase in COO concentration confirming grafting. There was no change in the crystal structure or % crystallinity of starch before and after grafting. Considerable increase in thermal stability with initial weight loss of 4% for grafted samples compared to 37% for the ungrafted samples was suggested to be due to the hydrophobicity imparted after grafting which was also confirmed by the contact angle measurements. In terms of the nanocomposites, the Tg of PLA did not show any major change but the cold crystallization temperature decreased from 124 to 113 °C when 30% modified starch nanocrystals were included. Addition of up to 5% modified starch nanocrystals did increase crystallization of PLA but the crystallization decreased at higher concentrations. Modulus of the composites showed marginal increase up to a nanoparticle concentration of 10% (Zamir et al. 2019). Instead of directly adding nanomaterials into PLA, a novel approach of combining cellulose nanocrystals with starch and poly(vinyl alcohol), electrospinning the blend and later adding the electrospun fibers into PLA to form films was followed. Addition of cellulose nanocrystals into starch nanofibers had improved the dispersibility of the polymers in PLA. Although starch and cellulose had poor crystallinity, there was no major influence on the crystallinity of the blend PLA films (Dicastillo et al. 2017). Marginal increase in strength and elongation were observed in the PLA films but agglomeration occurred at higher concentrations. Decrease in transparency and

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water vapor permeability were also attributed to the presence of electrospun fibers containing the cellulose nanoparticles (Dicastillo et al. 2017).

6 Nanoclay Reinforced PLA Composites Nanoclay of different dimensions and properties has been used to reinforce or improve performance of PLA. A nanoclay C30B was dispersed in dichloromethane and ultrasonicated for 3 h at 200 W and homogenized for 90 min. PLA solution was also predispersed in dichloromethane and combined with the nanoclay solution. The suspension was cast onto molds, cured at 23 °C for 16 h and later dried at 50 °C for 24 h to form the films. Compared to nanocellulose crystals or cellulose nanofibers made from sisal fibers, the clay reinforced PLA had inferior oxygen barrier, mechanical and also thermal properties (Fig. 16) (Trifol et al. 2016). Fig. 16 Comparison of the oxygen and water vapor permeability of PLA containing cellulose nanofibers, nanocrystals and nanoclay C30B (Trifol et al. 2016). Reproduced with permission from John Wiley and Sons

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Fig. 17 TEM images depicting the intercalating behavior of PLLA films modified with MMT (left), organically modified 350 (center) and C30B clay (right) (Fordiani et al. 2013). Reproduced with permission from John Wiley and Sons

Hence, hybrid nanocomposites were made using a combination of PLA, nanoclay and cellulose nanofibers. Clay used was an organo-modified montmorillonite (MMT) having particle size of 16–22 μm and modified using octadecylamine. Cellulose nanowhiskers were separated from microcrystalline cellulose using a sequential chemical extraction and ultrasonification approach. Extracted cellulose whiskers were combined with PLA and modified MMT and cast into films. PLA was found to have polar interactions with both clay and cellulose. Tensile properties of the hybrid films increased up to a cellulose content of 3% and ductility of the films also improved (10–90%). Although the nanoclay was well dispersed and intercalated with PLA, it was suggested that modifications to cellulose to improve interactions would probably lead to better film properties (Arjmandi et al. 2015a; b). A green modification approach was used to improve intercalation of nanoclay with PLA (Fordiani et al. 2013). To achieve the enhanced intercalation, a complex containing ulvan and glycoproteins was extracted from seaweeds and used an as additive to PLA and the improvement in properties was studied using C30B clay and MMT. TEM images (Fig. 17) showed considerably improved intercalation and less aggregation for the modified clay containing films compared to C30B or MMT containing films. It was found that the nanoclay improved crystallization kinetics and also caused decrease in interfacial energy leading to the better intercalation. In an attempt to increase the properties of hybrid PLA films, cellulose nanowhiskers obtained from MCC were modified through acid hydrolysis to obtain an average particle size of 50 μm and later combined with PLA and nanoclay. The hybrid constituents were combined together using chloroform and cast into films with different levels of nanoclay and cellulose (Arjmandi et al. 2016). Presence of cellulose nanowhiskers substantially increased the biodegradation of the samples and hence the weight loss. Increasing cellulose content caused higher degradation with a maximum weight loss of 2.5% after 8 weeks of burying the samples containing 9% cellulose. Similarly, thermal stability improved whereas activation energy decreased due to the addition of cellulose, particularly above 5%. It was suggested that the cellulose and PLA had polar interactions which led to increase in tensile strength from 18 to 36 MPa at 1% reinforcement. However, agglomeration of the nanowhiskers at higher concentrations leads to substantial decrease in strength. Elongation of the films also showed

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a similar trend with strength, but the modulus of the films continually increases with cellulose content. Nanoclay in the hybrid films showed good intercalation and assisted in the dispersion of the cellulose nanowhiskers. Presence of nanowhiskers reduced water vapor transmission but did not affect the transparency (Arjmandi et al. 2016). Although the presence of nanoclay was found to improve performance properties of hybrid PLA films, the role of nanoclay with respect to its ability to replace cellulose nanowhiskers was not clearly understood. When nanoclay content was progressively increased and cellulose content was decreased, it was found that highest tensile strength could be obtained at a cellulose content of 1% and nanoclay content of 4% with 79% improvement in ductility (Fig. 18). Higher nanoclay content increased thermal stability whereas cellulose increased biodegradability Arjmandi et al. 2015a, b).

Fig. 18 Influence of amount of nanoclay and cellulose nanowhiskers on the tensile properties of PLA films (Arjmandi et al. 2015a; b). Reproduced with permission from Springer Nature

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Table 10 Properties of films developed from CNF, PLA and their blends containing 1% casein as compatibilizer (Khakola et al. 2018). Reproduced with permission from Elsevier Parameter

CNF

CNF/PLA

CNF/PLA, 1% casein

1.21 ± 0.01

1.02 ± 0.02

1.08 ± 0.01

86 ± 6

1.6 ± 0.1

70 ± 5

78 ± 5

Tensile index (Nm/g)

99 ± 12

1.4 ± 0.0

69 ± 3

72 ± 5

Breaking strain (%) Elastic modulus (GPa) Tensile energy absorbed (J/g)

1.23 ± 0.01

PLA

Tensile strength (MPa)

Density

(g/cm3 )

1.8 ± 0.2 10.1 ± 0.6 1.1 ± 0.1

318 ± 13

2.3 ± 0.3

5.3 ± 0.3

0.01

5.6 ± 0.4

6.3 ± 0.3

3.3 ± 0.2

1.7 ± 01

2.7 ± 01

7 Protein Compatibilized PLA Nanocomposites Another approach to improve the compatibility between cellulose nanoparticles and PLA was attempted by using the milk protein casein (Khakola et al. 2018). Nanofibrillar cellulose was prepared by treating bleached softwood kraft pulp with sodium for defibrillation and later with mechanical refining. Casein proteins were dispersed in water and later treated with bicarbonate and heated for 30 min at 45 °C to cause thermal denaturation. Casein and cellulose nanofibril suspensions were combined with PLA latex and filtered to form a powder which was later compression molded at 80 °C, 1800 Pa for 2 h. Work of adhesion (52–60 mJ/m2 ) and pull-out force (372– 420 mJ/m2 ) showed considerable increase with the incorporation of casein into the PLA and cellulose nanofibrils containing films. Presence of casein increased the density of the composite films, decreased the number of voids which could promote better stress distribution between the matrix and reinforcement confirming that casein functioned as a compatibilizer. Consequently, considerable increase in tensile properties had occurred (Table 10). It was reported that casein compatibilized PLA could be used to develop 3D shaped cellulose-based packaging materials.

8 Conclusions and Future Perspectives Nanomaterials have proven to effectively improve the performance properties of poly (lactic acid) based materials. Cellulose based nanofibers, nanoparticles derived from wood and non-wood sources including bacteria have been predominantly used as reinforcement/fillers. Several authors have also developed hybrid PLA nanocomposites combining nanoclay, proteins or other additives to improve the compatibility between PLA and the additives. Similarly, chemical modifications such as grafting have also been adopted. Inclusion of nanomaterials has primarily led to increase in mechanical properties, decrease in water and oxygen transmission and even resistance to heat and UV radiation. However, use of nanofillers above the optimum concentration has led to decrease in properties including transparency, strength and

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modulus. Although most studies confirm some interaction between the nanomaterials and PLA and improvement in properties, suitability of the composites developed for specific applications has not been reported. Using available literature, additional studies have to be conducted to tailor the composites for targeted applications, particularly for food and medical applications due to the inherent advantages of PLA. Similarly, new processing techniques other than the conventional solution casting and injection molding should be studied.

References Arjmandi R, Hassan A, Haafiz MKM, Zakaria Z (2015) Partial replacement effect of montmorillonite with cellulose nanowhiskers on polylactic acid nanocomposites. Int J Biol Macromol 81:91–99 Arjmandi R, Hassan A, Eichhorn SJ, Haafiz MKM, Zakaria Z, Tanjung FA (2015) Enhanced ductility and tensile properties of hybrid montmorillonite/cellulose nanowhiskers reinforced polylactic acid nanocomposites. J Mater Sci 50(8):3118–3130 Arjmandi R, Azman Hassan A, Haafiz MKM, Zakaria Z, Islam MS (2016). Effect of hydrolysed cellulose nanowhiskers on properties of montmorillonite/polylactic acid nanocomposites. Int J Biol Macromol 82:998–1010 Chuensangjun C, Kanomata K, Kitaoka T, Chisti Y, Sirisansaneeyakul S (2019a) Surface-modified cellulose nanofibers-graft-poly (lactic acid) s made by ring-opening polymerization of l-lactide. J Polym Environ 27(4):847–861 Chuensangjun C, Kitaoka T, Chisti Y, Sirisansaneeyakul S (2019b) Chemo-enzymatic preparation and characterization of cellulose nanofibers-graft-poly (lactic acid)s. Eur Polymer J 114:308–318 Coltelli M-B, Cinelli P, Gigante V, Aliotta L, Morganti P, Panariello L, Lazzeri A (2019) Chitin nanofibrils in poly (lactic acid)(PLA) nanocomposites: dispersion and thermo-mechanical properties. Int J Mol Sci 20(3):504 Fordiani F, Aubry T, Pillin I, Grohens Y (2013) Enhancement of polylactide nucleation by montmorillonites intercalated with a green seaweed byproduct. J Appl Polym Sci 127(1):724–732 Fortunati E, Luzi F, Puglia D, Petrucci R, Kenny JM, Torre L (2015) Processing of PLA nanocomposites with cellulose nanocrystals extracted from Posidonia oceanica waste: innovative reuse of coastal plant. Indus Crops Products 67:439–447 Gan I, Chow WS (2019) Synthesis of phosphoric acid-treated sugarcane bagasse cellulose nanocrystal and its thermal properties enhancement for poly (lactic acid) nanocomposites. J Thermoplas Compos Mater 32(5):619–634 Gitari B, Chang BP, Misra M, Navabi A, Mohanty AK (2019) A comparative study on the mechanical, thermal, and water barrier properties of PLA nanocomposite films prepared with bacterial nanocellulose and cellulose nanofibrils. BioResources 14(1):1867–1889 Haafiz MKM, Hassan A, Khalil HPSA, Fazita MRN, Islam MS, Inuwa IM, Marliana MM, Hussin MH (2016) Exploring the effect of cellulose nanowhiskers isolated from oil palm biomass on polylactic acid properties. Int J Biol Macromol 85:370–378 Herrera N, Salaberria AM, Mathew AP, Oksman K (2016) Plasticized polylactic acid nanocomposite films with cellulose and chitin nanocrystals prepared using extrusion and compression molding with two cooling rates: effects on mechanical, thermal and optical properties. Compos A Appl Sci Manuf 83:89–97 Iglesias Montes ML, Luzi F, Dominici F, Torre L, Cyras VP, Manfredi LB, Puglia D (2019) Design and characterization of PLA bilayer films containing lignin and cellulose nanostructures in combination with umbelliferone as active ingredient. Front Chem 7:157

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Khakalo A, Filpponen I, Rojas OJ (2018) Protein-mediated interfacial adhesion in composites of cellulose nanofibrils and polylactide: enhanced toughness towards material development. Compos Sci Technol 160:145–151 Kian LK, Saba N, Jawaid M, Sultan MTH (2019) A review on processing techniques of bast fibers nanocellulose and its polylactic acid (PLA) nanocomposites. Int J Biol Macromol 121:1314–1328 Li C, Liu H, Luo B, Wen W, He L, Liu M, Zhou C (2016) Nanocomposites of poly (l-lactide) and surface-modified chitin whiskers with improved mechanical properties and cytocompatibility. Eur Polymer J 81:266–283 López de Dicastillo C, Roa K, Garrido L, Pereira A, Galotto M (2017) Novel polyvinyl alcohol/starch electrospun fibers as a strategy to disperse cellulose nanocrystals into poly (lactic acid). Polymers 9(4):117 Luzi F, Fortunati E, Jiménez A, Puglia D, Chiralt A, Torre L (2017) PLA nanocomposites reinforced with cellulose nanocrystals from posidonia oceanica and ZnO nanoparticles for packaging application. J Renew Mater 5(2):103–115 Mariano M, Pilate F, de Oliveira FB, Khelifa F, Dubois P, Raquez JM, Dufresne A (2017) Preparation of cellulose nanocrystal-reinforced poly (lactic acid) nanocomposites through noncovalent modification with PLLA-based surfactants. ACS Omega 2(6):2678–2688 Martínez-Sanz M, Lopez-Rubio A, Lagaron JM (2012) Optimization of the dispersion of unmodified bacterial cellulose nanowhiskers into polylactide via melt compounding to significantly enhance barrier and mechanical properties. Biomacromol 13(11):3887–3899 Miao C, Hamad WY (2016) In-situ polymerized cellulose nanocrystals (CNC)—poly (l-lactide) (PLLA) nanomaterials and applications in nanocomposite processing. Carbohyd Polym 153:549– 558 Panaitescu DM, Frone AN, Chiulan I, Gabor RA, Spataru IC, C˘as¸a˘ ric˘a A (2017)Biocomposites from polylactic acid and bacterial cellulose nanofibers obtained by mechanical treatment.BioResources 12(1):662–672 Piekarska K, Sowinski P, Piorkowska E, Haque MMU, Pracella M (2016) Structure and properties of hybrid PLA nanocomposites with inorganic nanofillers and cellulose fibers. Compos Part A Appl Sci Manuf 82:34–41 Scaffaro R, Botta L, Lopresti F, Maio A, Sutera F (2017) Polysaccharide nanocrystals as fillers for PLA based nanocomposites. Cellulose 24(2):447–478 Somord K, Somord K, Suwantong O, Thanomsilp C, Peijs T, Soykeabkaew N (2018) Self-reinforced poly (lactic acid) nanocomposites with integrated bacterial cellulose and its surface modification. Nanocomposites 4(3):102–111 Soykeabkaew (2016) Self-reinforced poly (lactic acid) nanocomposites of high toughness. Polymer 103:347–352 Takkalkar P, Griffin G, Kao N (2019) Enhanced mechanical and barrier performance of poly (lactic acid) based nanocomposites using surface acetylated starch nanocrystals. J Polymers Environ 1–11 Tingaut P, Zimmermann Y, Lopez-Suevos F (2009) Synthesis and characterization of bionanocomposites with tunable properties from poly (lactic acid) and acetylated microfibrillated cellulose Biomacromol 11(2):454–464 Trifol J, Plackett D, Sillard C, Hassager O, Daugaard AE, Julien B, Szabo P (2016) A comparison of partially acetylated nanocellulose, nanocrystalline cellulose, and nanoclay as fillers for highperformance polylactide nanocomposites. J Appl Polymer Sci 133(14) Yang W, Fortunati E, Dominici F, Giovanale G, Mazzaglia A, Balestra GM, Kenny JM, Puglia D (2016) Synergic effect of cellulose and lignin nanostructures in PLA based systems for food antibacterial packaging. Eur Polymer J 79:1–12 Ying Z, Defeng Wu, Wang Z, Xie W, Qiu Y, Wei X (2018) Rheological and mechanical properties of polylactide nanocomposites reinforced with the cellulose nanofibers with various surface treatments. Cellulose 25(7):3955–3971 Zamir SS, Frouzanmehr MR, Nagalakshmaiah M, Ajji A, Robert M, Elkoun S (2019) Chemical compatibility of lactic acid-grafted starch nanocrystals (SNCs) with polylactic acid (PLA). Polym Bull 76(7):3481–3499

Effect of Cellulose Nanocrystals on the Mechanical Properties of Polymeric Composites Matthew J. Dunlop, Bishnu Acharya, and Rabin Bissessur

Abstract The demands of modern society are increasingly driving a shift from nonrenewable petroleum-based materials to more sustainable renewable resources. As the most prevalent natural polymer on Earth, cellulose has attracted broad interest for use as a green additive or replacement for unsustainable petroleum-based materials. The cellulose polymer arranges naturally into microscale semi-crystalline cellulose fibrils which can be degraded, typically by chemical treatments, to yield the nanoscale crystalline region of the cellulose polymer. These nanoscale crystals display a high aspect ratio with length from 100 to 3000 nm and width from 3 to 50 nm. They also possess excellent mechanical properties, comparable to high cost synthetic reinforcing nanomaterials such as carbon nanotubes. The morphology of these cellulose nanocrystals (CNCs) is highly dependent on natural cellulose source material and the utilized CNC preparation conditions. When utilizing CNC as a reinforcing filler material within a polymer matrix, the resulting nanocomposite often displays properties dependent on the morphology of the filler. This chapter discusses the benefits, drawbacks and differences between CNC sources with focus on the resulting CNC morphology and its effect on the mechanical properties of CNC containing polymeric nanocomposites. In addition, recent findings in the area of hybrid multifiller CNC reinforced polymeric nanocomposites will be highlighted and contrasted with monofiller CNC polymeric nanocomposites. Keywords Cellulose nanocrystals · Polymeric nanocomposites · Mechanical properties · Hybrid

B. Acharya (B) Department of Chemical and Biological Engineering, University of Saskatchewan, Saskatoon S7N 5A9, Canada e-mail: [email protected] M. J. Dunlop · R. Bissessur Department of Chemistry, University of Prince Edward Island, Charlottetown C1A 4P3, Canada © Springer Nature Singapore Pte Ltd. 2021 M. T. Hameed Sultan et al. (eds.), Biocomposite Materials, Composites Science and Technology, https://doi.org/10.1007/978-981-33-4091-6_4

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1 Cellulose Background The natural polymer cellulose has been utilized by humanity for thousands of years and can still be commonly found today, as before, in numerous applications including wooden homes, paper books, and cotton-based clothing. Being the most abundant natural polymer on Earth, cellulose has been utilized for wide ranging applications across diverse demographics (Ummartyotin and Manuspiya 2015). Cellulose is commonly known as a linear polysaccharide with repeat units of cellobiose, which is comprised of β-1,4-glycosidic linkages between neighboring glucose residues. The extended nature of these linkages allows for the precise interaction of neighboring chains over long ranges to form a rigid and ordered structure. Therefore, cellulose does not generally occur naturally as a single polymer chain but is synthesized as a natural fiber from many cellulose polymer chains, described as cellulose fibrils (Fig. 1). The cellulose chains associate strongly in this way via both inter-chain and intra-chain hydrogen bonding between glucose residues. The precise manner of this interaction leads to cellulose fibers which are often largely crystalline.

Fig. 1 Schematic illustration of a natural cellulosic fiber and its structural components

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Cellulose is primarily found in forests where trees are thought to be the most important and abundant source, however cellulose can be found in all plants to various degrees (Beck-Candanedo et al. 2005). Additionally, cellulose can be found in bacteria (Roman and Winter 2004), algae (Revol 1982) and many other natural materials (de Rodriguez et al. 2006), (de Menezes et al. 2009). Interestingly there exists only a single animal source of cellulose, which is found in the marine invertebrate animal tunicate (Zhao and Li 2014). Regardless of the source, cellulose is almost always arranged in nature as either macro or microfibrils which contain crystalline regions of ordered cellulose polymer chains and amorphous regions with disordered cellulose polymer chains. All naturally synthesized cellulose is formed as Cellulose I which has two structures, the triclinic metastable cellulose Iα and the stable monoclinic cellulose Iβ crystal structure (Fig. 2). Natural cellulose can also be converted to Cellulose II, Cellulose III and Cellulose IV using various thermal or chemical treatments (Ciolacu and Popa 2011). The amount of cellulose present in a given cellulose containing material, the percentage of the cellulose which is crystalline, and the structural polymorph of the crystalline cellulose present is highly dependent on the specific cellulose source. Plant sources such as wood contain roughly 50% cellulose, of which only around 50% is crystalline, and the crystalline cellulose is arranged in both the cellulose Iα and the Iβ crystal structure (Park et al. 2010). Alternatively, the tunicate derived cellulose containing tunic is roughly 60% cellulose,

Fig. 2 a Schematic illustration of the difference between the triclinic cellulose Iα and monoclinic cellulose Iβ crystal structure. Typical cross-section of b) plant sourced and c) tunicate sourced CNCs. Adapted from Sugiyama et al. (1991) and Elazzouzi-Hafraoui et al. (2007)

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which is between 70 and 95% crystalline and arranged almost exclusively in the Iβ crystal structure (Berrill 1950). Of increasing interest today is the isolation of nanoscale cellulose crystals from natural resources and its utilization as a mechanical reinforcement in polymeric composites. To isolate these cellulose nanocrystals (CNC) researchers have used a variety of individual and combined methods, which generally fall under the categories of chemical treatment, mechanical treatment or enzymatic treatment (Klemm et al. 2011). Isolation methodologies are not the focus of this chapter and are described in more detail by others (Le Normand et al. 2014) (Jonoobi et al. 2015). However, given the wide range of natural cellulose sources and CNC isolation methods, it is apparent that CNC isolated from different sources with similar or different CNC isolation methodologies will yield CNC products with a wide range of morphologies. The focus of this chapter is to examine the effect of CNC morphology on the properties of CNC embedded polymeric nanocomposites. Particular attention will be paid to the mechanical properties of the polymeric nanocomposites, and how those properties change when the morphology of the CNC reinforcement is varied. To define a realistic scope for the chapter, discussion will be limited to CNC based polymeric composites where the CNC is in the form of discreet individual crystalline rods, also described throughout literature as cellulose whiskers or nanocrystalline cellulose (NCC).

2 Nanocellulose 2.1 Advantages at the Nanoscale First isolated by the French chemist Anselme Payen in 1839, cellulose is ubiquitous in modern society (Purves 1954). However, it was not until the early 1980s that nanocellulose materials began to draw broad interest from the scientific community (Turbak et al. 1983). Nanomaterials are broadly defined as materials with at least one dimension in the nanoscale. Nanocellulose therefore, simply represents some arrangement of cellulose with one or more nanoscale dimensions. One of the primary advantages of nanocellulose compared to bulk cellulose is its high specific surface area (Kim et al. 2002). This high surface area allows for a greater degree of interaction between nanocellulose and the polymeric matrices which the nanocellulose is embedded within. In this section, the different classes of nanocellulose will be briefly described with an emphasis on CNCs.

2.2 Classes of Nanocellulose There does not exist at this point a standardized nomenclature for describing nanocellulose materials. Generally, cellulose is known to contain amorphous and crystalline

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regions and nanocellulose is simply segments of the cellulose polymer with at least one dimension (typically the diameter) in the nanoscale. Nanocellulose which contains both amorphous and crystalline regions typically arrange into a fibrous structure which is commonly denoted as either nanofibrillated cellulose (NFC) or cellulose nanofibers (CNFs). When the amorphous cellulose regions are removed (typically by acid hydrolysis), what remains are discreet rod like cellulose crystals. These are referred to as nanocrystalline cellulose (NCC) or cellulose nanocrystals (CNCs). Likewise, cellulose of microscale dimensions and crystalline structure is referred to as microcrystalline cellulose (MCC). When microscale cellulose contains crystalline and amorphous regions, a fibrous structure results described as microfibrillated cellulose (MFC). Other terms and abbreviations have been used to describe cellulose sourced nanoscale and microscale materials throughout literature. In the context of this chapter however, we limit ourselves to discussing the effect of crystalline cellulose nanomaterials (CNC and NCC) on the mechanical properties of polymeric nanocomposites.

2.3 Cellulose Nanocrystals Reminiscent of the crystalline regions within elementary fibers of the natural cellulose source, CNCs are pure cellulose crystals generally shaped like whiskers or thin rods. They exhibit a large length compared to a relatively narrow width (Table 1), which contributes to the high aspect ratio (the length/width) of CNCs. The CNC morphology (aspect ratio, crystal structure, polydispersity and crystallinity) is highly variable and is dependent on the cellulose source, the CNC extraction process Table 1 CNC morphology and its variance with CNC source material and preparation conditions. L = CNC length and W = CNC width (diameter) Source

Preparation method

Length (nm)

Width (nm)

Aspect ratio (L/W)

References

Wood

H2 SO4 hydrolysis

100–300

3–5

20–100

Beck-Candanedo et al. (2005)

Cotton

HCl hydrolysis

100–150

5–10

10–30

Araki et al. (2001)

Ramie

H2 SO4 hydrolysis

70–200

5–15

~12

De Menezeset al. (2009)

Sisal

H2 SO4 hydrolysis

100–300

3–5

~60

De Rodriguez et al. (2006)

Tunicates

H2 SO4 hydrolysis

>1000

10–20

~100

Kimura et al. (2005)

Bacteria

H2 SO4 hydrolysis

100–1000

10–50

2–100

George and Bawa (2010)

Bacteria

HCl hydrolysis

160–420

15–25

7–23

George (2012)

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and the methods used to measure the morphological properties. Typical widths for CNCs are in the range 2–50 nm, and lengths between 100 and 3000 nm depending on the source material, processing conditions and measurement techniques (Habibi et al. 2010). These materials are highly crystalline, with percent crystallinity varying from 50 to 95% depending on the cellulose source, measurement technique and the data analysis method. Given their nanoscale dimensions, CNCs also exhibit high surface area in the range of 150–250 m2 g−1 (Shaheen and Emam 2018). The abundant hydroxyl groups present on the surface of CNCs allow for a diverse array of possible chemical modifications on the CNCs including etherification, oxidation, silylation, esterification, polymer grafting and others (Salas et al. 2014). This allows for hydrophilic CNCs to be incorporated into both hydrophilic and hydrophobic matrices, provided that the appropriate surface chemistry is imbued to the CNCs prior to incorporation within the polymer matrix of interest. The numerous CNC surface modifications available provide researchers with a diverse panoply of experimental approaches to incorporate CNCs into polymer matrices. By incorporating CNCs within a polymer matrix, properties of the resulting nanocomposite can be varied. CNCs are well known to alter the thermal, structural, and morphological properties of polymeric nanocomposites; however mechanical properties are often the most sensitive to CNC addition (Klemm et al. 2011).

2.4 CNC Commercial Production The production of cellulose worldwide is estimated to be in the range of 1010–1011 tons yearly (Azizi Samir et al. 2005). In order to meet the growing demand from academia, government and industry, the worldwide production of CNCs from this abundant cellulose stock is increasing in volume, and diverse cellulose sources are being utilized. It was recently proposed by ‘The Technical Association of the Pulp and Paper Industry’ (TAPPI) that in the year 2020 the gross world productions of nanocellulose materials may exceed $600 billion (Endes et al. 2016). There are numerous large scale production facilities capable of isolating kilograms of CNC from plant based sources. The largest is Canada based CelluForce©, a result of the partnership between FPInovations and the Domtar Corporation, which boasts a CNC production capacity of ~1000 kg CNC/day (Sharma et al. 2019). At the Forest Products Laboratory in Madison Wisconsin USA, CNCs are extracted from wood at a rate of ~10 kg/day (Chauve and Bras 2014). According to the 2016 report (Frost & Sullivan 2016), “Emerging Applications of Nanocellulose Technology”, several other private and public organizations are also producing CNCs at a ~10 kg/day rate including the Israeli company Melodea Ltd, the India Council for Agricultural Research, and Canada’s Blue Goose Biorefineries. In an effort to reduce the environmental impact of CNC isolation, (Mathew et al. 2014) used mild mechanical and chemical means to isolate ~600 g/day of CNCs from cellulose containing waste streams in a bioethanol producing pilot plant. All of these examples however rely on plant-based sources,

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primarily wood pulp, from which to extract the CNCs. Some production of bacterial CNCs (~17 g/day) (Chen et al. 2018) and algal CNCs (~36 g/day) (El Achaby et al. 2018; Masaoka et al. 2014; Masaoka et al. 1993) exist, however these are of much smaller scale than the plant based CNC sources. Tunicates likewise are converted to CNCs in very small, typically laboratory scale quantities of several grams or less (Jonoobi et al. 2015; Zhao and Li 2014). There is increasing interest in scaling this process up to a pilot plant scale (Dunlop et al. 2018), and recently the first known kilogram scale processing of tunicates to of CNCs was reported (Dunlop et al. 2020).

3 Sources of CNCs 3.1 Plants The primary sources of CNCs produced today are derived from plants, due in part to their natural abundance and accessibility. There is also a preexisting infrastructure in place to harvest and pretreat many plant sources of CNC. The best example of this is wood based CNCs, which are sourced primarily from the bleached Kraft wood pulp used conventionally to produce paper. Other examples include the packaging, pharmaceutical and textile industries which each have existing methods for the harvesting, pretreatment and processing of plant based CNC precursors (Moon et al. 2011). Generally the processing of plants to isolate plant based CNC is similar and consists of two main steps. Firstly, the removal of non-cellulose materials such as hemicellulose, lignin and other materials; followed by the removal of amorphous cellulose regions to yield individualized CNCs. An acid hydrolysis is typically used to remove amorphous cellulose regions, the most common being a sulfuric acid hydrolysis process. When sulfuric acid is utilized to extract CNCs, the surface of the nanocrystals is modified with sulphate groups. This imparts a negative surface charge to the CNC, increasing repulsion between the crystals, leading to a greatly increased CNC stability in an aqueous solution. In this way, numerous plants have been utilized for the extraction of CNCs, which include potato tubers (Dufresne et al. 2000), sugar beets (Leitner et al. 2007), ramie (Habibi and Dufresne 2008), cotton (Elazzouzi-Hafraoui et al. 2007), and many others (de Rodriguez et al. 2006, Helbert et al. 1996, Wang and Mohini 2007, Zuluaga et al. 2007). Plant sourced CNCs contain a mixture of cellulose Iα and Iβ crystal structures; with higher order plants such as trees and shrubs containing more cellulose Iβ, and lower order plants such as fungi and algae containing more cellulose Iα (de Souza Lima et al. 2004).

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3.2 Tunicates Named for their ‘tunic like’ cellulose containing epidermis, tunicates are unique among animals in their ability to biosynthesize cellulose (Zhao and Li 2014). The cellulose produced in tunicates is arranged in microfibrils and acts as a reinforcing skeletal-type network structure; this provides rigidity and support to the remaining protein and lipid materials which comprise the tunic (Kimura and Itoh 2007). In 2002 the genome of the common ascidian tunicate ‘Ciona intestinalis’ was sequenced for the first time, in part to enhance understanding of cellulose biosynthesis and function in tunicates (Dehal et al. 2002). Over recent years, tunicates have been increasingly viewed as an attractive source for the production of CNCs (Zhao and Li 2014; Dunlop et al. 2018). Primarily, this is due to the unique properties which tunicate CNCs exhibit compared to plant sourced CNCs. Tunicate CNCs exhibit the highest aspect ratio (~70–100) of all CNC sources. Additionally, tunicate derived CNCs are predominantly the cellulose Iβ allomorph, rather than the mixture of the cellulose Iα and the Iβ crystal structure displayed in plant sourced CNCs. Similar to plant sourced CNC, the isolation of tunicate sourced CNC occurs in two general steps. Firstly, the cellulose containing tunic is cleaned, deproteinated and bleached to remove the non cellulose tunic components. Next the purified tunic cellulose, referred to as tunicin, is hydrolyzed in acid to yield tunicate CNCs. As with plant CNCs, sulfuric acid is the most common acid used for the hydrolysis process. This leads to CNCs with surface sulfate groups that reduce agglomeration and increase the colloidal stability of the dispersed CNCs.

3.3 Other Sources Numerous other cellulose sources exist, which can be utilized as a source of CNCs. Some species of bacteria have the ability to biosynthesize and secrete cellulose microfibrils. Bacterially synthesized microfibrils display a high aspect ratio, typically higher than 50 (Brown and Laborie 2007). The exact bacteria and the growing conditions utilized will determine the morphology of the isolated CNCs. One common bacteria known as Acetobacter produces highly crystalline cellulose microfibrils with a cellulose Iα crystal structure (Tokoh et al. 1998). However, by altering the culturing conditions, the width of the microfibrils and the ratio of cellulose Iα to cellulose Iβ can be tailored (Yamamoto and Horn 1994). Interestingly, when some species of algae are treated in acid, the cellulose microfibrils present in the algal cell wall can be hydrolyzed to form CNCs. Like with other cellulose sources, the properties of isolated CNCs are dependent on the natural source material and preparation conditions. The algae Micrasterias and Valonia provide good examples of the contrast in CNC properties across algae species. Micrasterias CNCs are primarily cellulose Iβ and exhibit a rectangular cross-section (Hanley et al. 1997), whereas Valonia CNCs are primarily cellulose Iα and exhibit a square cross-section (Revol 1982). Cellulose

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isolated from Valonia has been shown to display exceptional crystallinity, in some cases exceeding 95% crystallinity (Sugiyama et al. 1991).

4 Composites Cellulose based composite materials date back thousands of years and have served many functions. Due to improvements in technology and scientific advancement, we are able today to prepare cellulose based nanocomposite materials. Given that a wide variety of cellulose based nanomaterials currently exist with diverse morphologies, we will narrow the focus of this chapter to cellulose based nanocomposite materials whose composition is limited to CNCs incorporated within a polymeric matrix. Since all cellulose sources yield CNCs which are highly polydisperse, and the range of naturally sourced CNC morphologies is highly variable; we will further simplify our composites into three main classes. i. Low aspect ratio plant CNC based polymeric nanocomposites ii. High aspect ratio tunicate CNC based polymeric nanocomposites iii. Hybrid CNC mixtures in polymeric nanocomposites Throughout the literature, researchers have used low aspect ratio CNCs from plantbased sources to prepare polymeric nanocomposites. However due to the nature of the plant based cellulose source, CNC isolation process, and characterization technique; these CNCs cover a wide range of aspect ratio distributions, different shapes in crosssection, changing ratios of cellulose Iα to Iβ and so on. Likewise, for high aspect ratio tunicates, a range of morphologies are reported throughout literature and these vary for similar reasons as plant-based CNC sources. We feel, given the variability in CNC morphology across natural sources and the inconsistencies in CNCs used between literature reports, that it is reasonable to separate CNC based composites into these three previously mentioned classes, visualized in Fig. 3. This will allow for simple comparison between the effect of high aspect ratio, low aspect ratio and hybrid mixtures of high and low aspect ratio CNCs, on the mechanical properties of polymeric nanocomposites.

4.1 Mechanical Reinforcement of CNCs in Polymeric Nanocomposites The mechanical reinforcement observed in CNC reinforced polymeric nanocomposites is thought to arise from two main factors. The first and most obvious factor being the impressive mechanical properties which CNC crystals display. Comparable to high cost, synthetic reinforcing nanomaterials such as carbon nanotubes (Moon et al. 2011), CNCs display a Young’s modulus from 110 to 220 GPa and tensile strength

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Fig. 3 Visualizing the incorporation of long high aspect ratio tunicate derived CNCs, short low aspect ratio plant derived CNCs and hybrid mixtures of high and low aspect ratio (long and short) CNCs within polymeric nanocomposites

in the range of 7.5–7.7 GPa (Lin et al. 2012). The second factor relates to the stress transfer between the polymer matrix and CNC filler. Strong interactions between matrix and filler such as covalent linkages or noncovalent interactions like hydrogen bonding allow for stress to be transferred from the viscoelastic polymer matrix to the elastic CNC reinforcement. The weaker the interaction between matrix and filler, the weaker the stress transfer from matrix to filler, leading to reduced reinforcement and lower nanocomposite mechanical properties (Kargarzadeh et al. 2017) (Dunlop et al. 2019). The first factor regarding superb individual CNC mechanical properties is largely consistent regardless of the CNC source or processing conditions. However, increased variability is observed when stress transfer is considered. Stress transfer can vary with numerous factors such as the surface chemistry of the filler and matrix, the nanocomposite preparation methodology, the presence of plasticizers or solvents, aggregation of the CNCs and the CNC morphology among others (Klemm et al. 2011). In short, any factor that modifies the formation or final structure of the percolating CNC network within the polymeric matrix will affect the mechanical properties of the resulting nanocomposite (Dufresne et al. 2000, Dufresne 2008). When fitting experimentally determined mechanical properties to expected mechanical properties, two general models are most often used: the percolation model (Takayanagi

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et al. 1964) and the Halpin-Kardos model (Halpin and Kardos 1972). The percolation model predicts the modulus of a nanocomposite material by assuming that the polymer matrix is too soft to contribute to the nanocomposite modulus, which is instead dominated by an interconnected or ‘percolating’ network of stiff filler material that has a continuous order and structure throughout the nanocomposite deriving from strong filler-filler interactions. Conversely the Halpin-Kardos model assumes a perfect distribution of the filler within the matrix polymer with no filler-filler interactions present. Both models are dependent on the average aspect ratio (length/width) of the CNCs. The percolation model predictions in particular are heavily dependent on the aspect ratio of the CNC filler material, with high aspect ratio fillers percolating the polymeric matrix and providing mechanical enhancements at lower loading levels (Peng et al. 2011). An illustration of the effect of CNC aspect ratio on nanocomposite properties is illustrated by the work of (Azizi Samir et al. 2005). In this study three CNC types were dispersed within a poly(S-co-BuA) polymer matrix to yield nanocomposite materials. The nanocomposite reinforced with high aspect ratio (~67) tunicate CNCs showed the greatest modulus increase and thermal stability compared to the neat polymer matrix. The bacterial sourced CNCs with aspect ratio (~60) showed the second greatest modulus and thermal improvements, followed by the low aspect ratio (~10) plant sourced CNCs. Notably, the aspect ratio dependence of the percolation threshold for a rigid rod like reinforcement within a polymeric matrix has been demonstrated to extend to various rigid filler materials such as carbon nanotubes (Wong et al. 2009) and others (Jiang et al. 2007). In this section the effect of CNC morphology is investigated as it relates to the mechanical properties of CNC containing polymeric nanocomposites.

4.2 Nanocomposites Based on Plant Sourced CNCs Plant sourced CNCs are short, generally low aspect ratio, crystals which contain a mixture of the cellulose Iα and the Iβ crystal structure. Higher order plants tend to exhibit higher degrees of the cellulose Iβ allomorph whereas lower order plants contain more of the cellulose Iα allomorph. There exist a plethora of reports and reviews on numerous aspects of plant CNC based polymeric nanocomposites (Moon et al. 2011; Kim et al. 2015; Oksman and Sain 2005). Primarily this is due to the prevalence, low cost and commercial availability of plant sourced CNCs compared to other sources. As with all CNC reinforced polymeric nanocomposites, the mechanical behavior of these materials are dependent on the aspect ratio of the CNCs, however plant sources of CNC generally exhibit a narrow and low aspect ratio distribution (de Rodriguez et al. 2006). Recently Hendren et al. combined wood derived CNCs with poly (5-triethoxysilyl-2-norbornene) (PTESN) to form nanocomposite solvent cast films (Hendren et al. 2019). When the mechanical properties of the films were probed it was found that the Young’s modulus and storage modulus increases as CNC loading is increased. At 20% CNC addition the Young’s modulus almost doubled from 540 to 970 MPa and the storage modulus tripled from 400 to 1200 MPa. In another recent

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study, (Chen et al. 2019) compared starch-based bionanocomposite films which incorporated nanocellulose isolated from several different non wood fibers. Of the various nanocellulose materials tested, the bamboo sourced nanocellulose displayed the best reinforcement at the lowest filler content. This was primarily ascribed to the aspect ratio of the bamboo sourced nanocellulose being the highest of all the nanocellulose studied. In an effort to further improve the mechanical properties of polymeric nanocomposites containing plant sourced CNCs, some authors have begun to incorporate polymeric dispersants into these composite materials in an effort to limit CNC aggregation and increase the stress transfer between polymeric matrix and CNC reinforcement (Meesorn et al. 2017). It has been demonstrated for poly (ethylene oxideco-epichlorohydrin), polyurethane and poly(methyl acrylate) matrices, that by using polyvinyl alcohol as a polymeric dispersant for CNCs, significant improvements in mechanical properties are achievable (Meesorn et al. 2017) (Miri et al. 2016). This is thought to arise from improved dispersion and reduced aggregation of the CNC filler material. By taking advantage of the hydrophilic nature of the polyvinyl alcohol and CNCs, (Meesorn et al. 2019) was able to prepare a polymeric nanocomposite from poly (ethylene oxide-co-epichlorohydrin), polyvinyl alcohol and CNCs which displayed tunable mechanical properties with humidity level. When dry the polymer matrix displayed a storage modulus of 2.7 MPa, rising to 50 MPa when reinforced with 10% wt. plant CNCs. Interestingly when 5% wt. of polyvinyl alcohol is added to the same nanocomposite system the resulting storage modulus is 200 MPa, four times higher than without CNCs and almost 75 times higher than the unmodified polymer matrix. Notably, the Young’s modulus and tensile strength also increased when polyvinyl was added as a polymeric dispersant. When wet a similar trend is observed however the storage modulus is significantly lower, primarily due to hydrogen bonding competition between the composite components and the aqueous solution (Rusli et al. 2010).

4.3 Nanocomposites Based on Tunicate Sourced CNCs Tunicate sourced CNCs are long high aspect ratio crystals comprised almost exclusively of cellulose Iβ. The first study of polymeric nanocomposites comprising tunicate CNCs was reported by (Favier et al. 1995) who combined these nanoscale crystals with the polymer latex. This resulted in a nanocomposite material with improved mechanical properties at low CNC content, particularly when the nanocomposite was heated beyond the glass transition temperature of the polymer matrix. Improved mechanical properties were also documented for a composite system comprised of amorphous poly(β-hydroxyoctanoate) and tunicate derived CNCs, the reinforcement documented by (Dubief et al. 1999) resulted from the formation of a rigid network of high aspect ratio CNC filler material. By combining either of the hydrophobic matrices poly(styrene-co-butadiene) or polybutadiene with hydrophilic tunicate derived CNCs, Dagnon et al. were able to prepare a biomimetic and stimuliresponsive polymeric nanocomposite (Dagnon et al. 2012). At 17% by volume,

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the tunicate CNCs enhanced the storage modulus of the nanocomposites three-fold compared to the neat polymer matrix. Above the glass transition the mechanical enhancements in storage modulus were similar to the results of Favier et al., where CNCs act as rigid stabilizers for the glassy state polymer matrix. The authors noted that even when swollen in aqueous media, the mechanical reinforcement which CNCs imbue to the polymeric nanocomposites is preserved, however the overall mechanical properties of all samples is reduced. This was determined to result from the tunicate CNCs acting as hydrophilic channels which aid in the uptake of water molecules which solvate the CNCs and limit their stress transfer to the polymer matrix while lessening the strength of CNC filler-filler interactions. These findings were built on by (Jorfi et al. 2013) who prepared physiologically responsive and mechanically adaptive polymeric nanocomposites for eventual use in biomedical applications.

4.4 Hybrid Nanocomposites comprised of Multiple CNC Sources In recent years, the production of hybrid polymeric nanocomposite materials, which combine multiple nanoscale filler materials within the same polymeric matrix, has gained significant interest. The properties of hybrid polymeric nanocomposites depend primarily on the polymer and nanoscale fillers used. Interestingly, it has been demonstrated that hybrid polymeric nanocomposite materials which utilize nanoscale fillers of different morphologies, often show mechanical properties which exceed monofiller nanocomposites at the same filler loading level (Sapkota et al. 2017, Yeng et al. 2011, El Miri et al. 2016). This indicates that there exists a cooperative interaction between nanoscale fillers of different morphology, which leads to these enhanced properties. Herein we will discuss hybrid polymeric nanocomposites in the context of highly elastic rod like nanomaterials with different aspect ratios (long and short CNCs), being used to provide reinforcement in a soft polymer matrix. There exists a small but growing body of work in this area which highlights the growing potential of, and unique advantages to, using long and short rod like nanomaterials within a common polymer matrix. As discussed previously, numerous researchers have reported polymeric nanocomposite materials which contain either plant sourced or tunicate sourced CNCs (Meesorn et al. 2017, Samir et al. 2005). Across these reports, the difference in morphology (aspect ratio) of the CNC reinforcement is primarily credited with the differences in mechanical properties observed between nanocomposite materials reinforced with CNCs of a different morphology but the same volume fraction (Jorfi et al. 2013). The large aspect ratio of tunicate CNCs allow them to percolate and form an interconnected network within the polymer matrix at a lower concentration than CNCs with a lower aspect ratio. The structure and orientation of this percolation network determines in large part the stress transfer between the soft polymer matrix and the ridged reinforcing filler material (Šturcová et al. 2005).This percolating

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network is known to change its structure and orientation based on the morphology of the filler materials. Using Raman spectroscopy techniques, (Rusli et al. 2011) demonstrated that enhanced stress transfer from CNC filler to epoxy polymer matrix was observed for tunicate CNCs compared to wood CNCs. The authors attributed this difference in stress transfer to the changing morphology of the CNC reinforcement. A pioneering 2011 study by (Yeng et al. 2011) combined nanoscale graphene sheets and carbon nanotubes within an epoxy resin. Surprisingly, the authors found that hybrid combinations of the two filler materials which display very different morphologies displayed a synergistic reinforcing effect within the epoxy. The tensile strength of the hybrid nanocomposites increased by over 35% compared to the neat epoxy, whereas the monofiller graphene containing nanocomposite displayed roughly 1% improvement in tensile strength. Building off this study, (El Miri et al. 2016) prepared polyvinyl alcohol based nanocomposite materials which incorporated a hybrid combination of plant sourced CNCs and graphene oxide nanosheets. The authors found synergistic behavior between the two nanoscale filler materials within the polyvinyl alcohol polymer matrix. This resulted in improved mechanical performance for hybrid multifiller materials compared to the neat polymer matrix or monofiller nanocomposites with the same volume fraction of reinforcement. The Young’s modulus and ultimate tensile strength of the hybrid nanocomposite increased 320% and 124% respectively compared to the neat polymer matrix; the CNC reinforced monofiller nanocomposite only increased by 77% and 12%, respectively. The initial study which incorporated both low aspect ratio plant and high aspect ratio tunicate sourced CNCs together within the same polymer matrix was accomplished by (Sapkota et al. 2017). They reported an aspect ratio dependence on the percolation threshold and proposed a modified percolation model, based on the aspect ratio distribution of the CNC source material, to predict experimental mechanical performance for this class of hybrid polymeric nanocomposites. Additionally the authors noted that the experimental modulus of the polymeric nanocomposite material is strongly weighted towards the contribution of high aspect ratio filler material with little effect from the low aspect ratio filler material. Recently, our group built on the research by (Sapkota et al. 2017) and (El Miri et al. 2016) and prepared polymeric nanocomposite materials with plant and tunicate derived CNCs incorporated within a polyvinyl alcohol matrix. We found that not only does the tunicate derived CNCs display higher reinforcement than plant CNCs, but when the high and low aspect ratio CNC fillers are combined in a 1:1 ratio they display cooperative behavior with respect to the elastic modulus of the nanocomposites. We determined that hybrid CNC mixtures display higher elastic modulus than either the plant or the tunicate containing monofiller nanocomposites, at the same filler content (Dunlop et al. 2019). This was attributed to a modification of the percolating network resulting in increased stress transfer to the polymer matrix, which was supported by an increase in the structural order of the nanocomposite materials as revealed by X-ray diffraction.

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5 Effect of Morphology on Mechanical Performance Generally, when discussing CNC morphology researchers report average lengths, widths and aspect ratio based on some form of statistical analysis, usually derived from experimental micrographs. It is important to consider when discussing the aspect ratio dependence of CNC imparted reinforcement in polymeric nanocomposites, that CNCs aspect ratios reported in literature are averages with corresponding polydispersity. Interestingly, when comparing CNC sources the polydispersity of the CNCs seems to scale with the average aspect ratio of the CNCs (Azizi Samir et al. 2005) (Natarajan et al. 2018). Currently, the consensus is that it is the higher aspect ratio of the CNCs which leads to the enhanced stress transfer in polymeric nanocomposites (Azizi Samir et al. 2005; Sapkota et al. 2017). For this reason, tunicate CNCs were shown to carry stress to a greater degree than low aspect plant CNCs by (Šturcová et al. 2005; Rusli et al. 2011). It should be noted however that the tunicate CNCs are also significantly more polydisperse than plant sourced CNCs (de Souza Lima et al. 2004). To the best of our knowledge, in the current literature CNC aspect ratio is generally considered the main factor in the expected reinforcement of polymeric matrices (Azizi Samir et al. 2005; de Souza Lima et al. 2004; Sapkota et al. 2017). Recently however, hybrid CNC nanocomposites have shown surprisingly high mechanical properties, even surpassing tunicate derived CNC reinforcement at the same CNC content (Fig. 4). The average aspect ratio of the hybrid CNC mixture is lower than the tunicate CNCs, and yet as filler the hybrid CNCs show superior mechanical properties than

Fig. 4 DMA stress versus strain overlay of hybrid nanocomposites at increasing CNC content a), and b) measured Young’s modulus for all composites including monofiller P-NCC-PVA and T-NCC-PVA as well as the hybrid T-P-NCC-PVA and ST-P-NCC-PVA

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the monofiller tunicate CNCs. This unusual result for hybrid nanocomposite materials could potentially be explained by the increased polydispersity of CNCs in the hybrid nanocomposite case, rather than solely considering the average aspect ratio of the multifiller CNC reinforcement. More detailed studies which investigate the use of highly polydisperse hybrid multifiller CNC reinforcements in polymeric composites are needed to better understand the unique mechanical properties which these materials exhibit. Future work in this area will further elucidate whether CNC reinforcement in polymeric composites is in part polydispersity dependent or is primarily dependent on the aspect ratio of the CNC filler.

6 Conclusion and Future Perspective Clearly there exists a strong and growing interest in the use of CNCs to modify the properties of polymeric matrices. Among the most important of these is the effect of CNCs on the mechanical properties of the corresponding polymeric nanocomposite material. CNCs from any source with any morphology have the potential to reinforce polymeric matrices, provided that the CNC and polymer share compatible surface chemistry and that the polymers elastic modulus is lower than the CNC. It has been shown that high aspect ratio CNCs, such as those sourced from tunicates; reinforce polymers at a lower CNC concentration than low aspect ratio CNCs such as those sourced from plants. When plant and tunicate CNCs are combined within the same polymer matrix, the resulting hybrid polymeric nanocomposites can display mechanical properties superior to either plant or tunicate CNCs individually dispersed within the polymer matrix. These findings have brought into question the notion that the aspect ratio of CNCs is the primary morphological factor when determining CNC reinforcement in polymeric matrices. We posit that the polydispersity of the CNC reinforcement may also play an important factor, and expect that future work towards understanding the effect of CNC morphology on the mechanical properties of monofiller and hybrid multifiller polymeric composites will be explored.

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Lignocellulosic Fiber-Reinforced PLA Green Composites: Effects of Chemical Fiber Treatment Farkhondeh Hemmati, Tara Farizeh, and Jamshid Mohammadi-Roshandeh

Abstract Bio-based polymer composites have been introduced as the most promising alternatives for the petroleum-based polymer composites, owing to the depletion of fossil fuel resources, global energy crisis and growing concerns about environmental pollutants. Among them, the green biocomposites totally based on plants, which consist of lignocellulosic fibers as reinforcement agents and biopolymers as matrices, have many beneficial advantages in a wide range of applications such as availability, renewability, eco-friendly, biodegradability and low density. The most frequently used green composites are the plant fiber-reinforced poly(lactic acid) (PLA) biocomposites. Regardless to the sustainability and ecologically friendly properties, PLA/plant fiber biocomposites must be modified to gain a comparable performance with the petroleum-based composites. Different chemical, physical, physiochemical and biological modification methods have been developed to treat the lignocellulosic fibers for improving the matrix/filler interfacial adhesion and biocomposite final properties. Through the literature, the most widely applied methods are the fiber chemical modifications. In this chapter, the effects of different chemical treatments of plant fibers, as the most feasible modification processes, in the PLA-based biocomposites have been discussed and the importance of choosing proper reaction conditions on controlling the biocomposite performance is clarified. Keywords Poly (lactic acid) · Plant fibers · Biocomposites · Chemical treatment

1 Introduction In recent decades, biodegradable polymers have been considered as promising substitutes for the petroleum-based polymers owing to the growing environmental awareness and worldwide concerns about the global warming and plastic pollutants (Sharif et al. 2019; Siakeng et al. 2019a). Using biodegradable plastics may alleviates the environmental problems caused by the large amounts of non-biodegradable plastic F. Hemmati (B) · T. Farizeh · J. Mohammadi-Roshandeh Caspian Faculty of Engineering, College of Engineering, University of Tehran, Guilan, Iran e-mail: [email protected] © Springer Nature Singapore Pte Ltd. 2021 M. T. Hameed Sultan et al. (eds.), Biocomposite Materials, Composites Science and Technology, https://doi.org/10.1007/978-981-33-4091-6_5

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wastes from huge daily consumptions of humans (Singhvi et al. 2019). Recently, poly(lactic acid) (PLA) or polylactide, among different biodegradable polymers, has been attracted a great attention as the most promising substitute for the fossil fuelbased polymers (Avérous 2008; Singhvi et al. 2019). In fact, this biopolymer is a renewable and compostable thermoplastic aliphatic polyester, which has contributed to the global plastic market, due to the reasonable price and favorable properties (Saeidlou et al. 2012). With repeating unit of C3 H6 O3 , PLA is a nontoxic, biocompatible and bio-based polymer that can be completely produced from the annual renewable resources such as corn starch and sugar beets (Avinc and Khoddami 2009; Ding et al. 2018; Nofar et al. 2019). The production process of PLA requires about 50% less non-renewable energy in comparison with the synthetic process of common synthetic plastics like polypropylene (PP), polyethylene (PE), polystyrene (PS), etc. (Ding et al. 2018). This ecofriendly polyester has high modulus and good transparency, comparable with PE, PS and poly(ethylene terephthalate) (PET), and can be shaped as fibers, films and household goods using common melt-processing machineries (Saeidlou et al. 2012; Sharif et al. 2019). Although PLA has been recognized as a proper substitute for synthetic polymers in packaging, electronics, non-woven fibers and household products, most of its primary applications have been limited to the biomedical field (Sharif et al. 2019). Appropriate mechanical properties, good biocompatibility and biodegradability, non-toxicity and thermal plasticity of PLA make it suitable for sutures, blood vessels, drug-delivery systems, resorbable implants and tissue scaffold in this field (Savioli Lopes et al. 2012). It is worth noting that the limitation of the primary uses of PLA in the biomedical applications was resulted from the low availability, high cost of production and low molecular weight (M w ) of the synthesized biopolymer. However, the recent developments in the dextrose fermentation process of corn starch has eventuated in the cost reduction of lactic acid (LA), which is the building block of PLA chains (Avinc and Khoddami 2009). Hydrolytic degradation of PLA macromolecules converts the chains into the LA monomers (Ding et al. 2018). There are different synthesis methods for the PLA production. Some of them have been applied in industrial scales.

2 PLA Synthesis Methods and Properties 2.1 PLA Synthesis Synthesis of PLA, usually begins with producing LA monomer and finishes with polymerization of an intermediate material known as lactide (Singhvi et al. 2019). Overall, there are two feedstocks for the LA production. One of them is the petroleum resources by which LA is obtained through the chemical synthesis route. The other one is the natural feedstock by which LA is synthesized through the carbohydrate fermentation process. The biotechnology method of LA production provides more

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that 90% market share (Hamad et al. 2015). The reasons behind paying special attention to the natural feedstock can be traced to the less dependence on the fossil fuels, less greenhouse gas emissions, use of biocatalysts and the possibility of producing LA with higher purity (Singhvi et al. 2019). LA can be synthesized by the bacterial fermentation of the food crops such as corn, wheat, rice, potato, sugar beet, sugar cane or agricultural wastes such as whey and cellulosic biomass (Ahmed and Varshney 2011).This method of synthesis mostly leads to an optically active enantiomer of LA, called Levo-lactic acid (L-LA) (Hamad et al. 2015). Nonetheless, the chemical route results in a racemic mixture of two optically active isomers of LA, Levo- (L-LA) and Dextro-lactic acid (D-LA), which can be considered as the main drawback of this method (Singhvi et al. 2019). As a matter of fact, Lactic acid or 2-hydroxypropionic acid (CH3 CHOHCOOH) has a chiral carbon atom and can be synthesized to form L or D optical active enantiomers (Sharif et al. 2019). A mixture of approximately 99.5% L-form and 0.5% D-Form yields from the bacterial fermentation. The properties of L and D enantiomers are similar, except for the rotation direction of the plane polarized light. The rotation of the polarized light for L-LA isomer is clockwise, whereas the rotation for D-LA is counterclockwise (Avinc and Khoddami 2009). The chemical structures of the possible stereoisomerisms of LA and natural fermentation route of LA is schematically shown in Fig. 1. Undoubtedly, the ratio and distribution of these stereoisomers along the PLA backbone substantially change the properties and processability of the biopolymer (Hassan et al. 2013). As it is also illustrated in Fig. 1, there are three common procedures for the PLA synthesis: (I) polycondensation of the LA monomer and increasing the PLA molecular weight using coupling agents, (II) ring-opening polymerization (ROP) of LA cyclic dimer, and (III) direct methods including azeotropic dehydration polycondensation and enzymatic polymerization (Savioli Lopes et al. 2012). In the first route (polycondensation), the condensation reaction of the bifunctional LA monomer is performed under vacuum and at high temperatures. However, the method just yields to the PLA prepolymers with low to moderate molecular weights (M w = 10,000–20,000 g/mol) owing to the problems in the water removal and impurities. This method is feasible through solution and melt polymerization processes. It is a single-stage process, fairly economical and easy to control. However, the low molecular weight of the synthesized PLA makes it inadequate for the plastic industrial applications. Using chain extenders or esterification-promoting agents is a common solution to modify the low molecular weight of PLA chains (Singhvi et al. 2019). One may keep in mind that this modification leads to higher costs and complexity of the synthesis process. Moreover, the presence of active chainextending agents, oligomers and metallic impurities, which are applied as catalysts, are the main disadvantages of this synthesis method (Avérous 2008). The mentioned second procedure for the PLA synthesis is the most widely used method for producing the PLA chains with high molecular weight. Because of the importance and prevalence of this method, PLA is also called polylactide. ROP reaction includes three stages: (I) condensation polymerization of LA, (II) depolymerization of PLA to the LA cyclic dimer or lactide and (III) ring-opening polymerization

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Fig. 1 Schematic drawing for the synthesis of lactic acid, lactide and PLA (Avérous 2008; Avinc and Khoddami 2009; Sharif et al. 2019)

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of lactide (Ding et al. 2018; Singhvi et al. 2019). The PLA synthesis through ROP procedure is relatively mild and has higher rates. Nevertheless, the need for the additional separation and purification process of lactide makes ROP method more complex and expensive. It must be noted that catalysts based on metals such as tin, aluminum, lead, zinc and etc. must be employed in ROP reaction, which their residual traces are not approving in some application fields such as biomedical or food industries (Avinc and Khoddami 2009; Singhvi et al. 2019). Since an asymmetric carbon center is present in LA, the synthesis of Lactide can lead to different stereoisomeric forms including D-lactide, L-lactide and meso-lactide (Avinc and Khoddami 2009). Possible stereochemistry configurations of lactide are also shown in Fig. 1. The PLA polymers synthesized purely from L-lactide or D-lactide are called poly(L-lactic acid) (PLLA) and poly(D-lactic acid) (PDLA), respectively. The ROP polymerization of L-lactide and D-lactide mixture with minimum content of 10% of one the stereoisomers bring about an amorphous PLA (Ding et al. 2018). A wide range of PLA polymers with completely different properties can be synthesized through the ROP procedure. This can be achieved by controlling the ratio and sequences of the stereoisomeric units of L- and D-lactide in the synthesized PLA linear macromolecules through changing the reaction temperature and time and the catalyst type and composition (Avinc and Khoddami 2009; Siakeng et al. 2019a). The ROP synthesis of PLA is possible through different polymerization methods including solution, bulk, melt and suspension polymerization processes. However, in industrial scale, it is preferred to apply the bulk and melt polymerization processes by using lower amounts of non-toxic catalysts (Avérous 2008). The third method mentioned in the feasible PLA synthesis procedures is a direct route for the PLA synthesis having high molecular weight. This method is more eco-friendly and necessitates mild reaction conditions. The enzymatic synthesis method of PLA yields polyester chains with proper structures by using low cost raw materials (Singhvi et al. 2019). Although direct methods based on azeotropic dehydration polycondensation and enzymatic polymerization have some advantages, but more researches are required for this synthesis route to be commercialized (Siakeng et al. 2019a). As mentioned before, the optical isomers of the PLA building blocks provide great opportunities to synthesize a wide range of PLA grades with different microstructures and properties by controlling the stereochemistry of the chain repeating units.

2.2 PLA Properties One may know that the stereochemistry of the macromolecular repeating units determines the crystallinity degree, crystal morphology, mechanical performance, degradation rate and thermal properties of PLA (Ding et al. 2018). In fact, completely amorphous or semi-crystalline biodegradable PLA can be synthesized by controlling the composition and sequence of the optically active isomers in the chain backbones. Moreover, the PLA crystallinity behavior depends on the molecular weight,

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branching, thermal history and additives (Ahmed and Varshney 2011). It can be seen in Fig. 2, the ratio of L and D stereoisomers and distribution of them across the chain backbone along with the polymerization degree determine the melting temperature (Tm ) and glass transition temperature (Tg ). For instance, a highly crystalline PLA polymer can be synthesized from a combination of L- and D-lactide monomers with less than 2% of D isomer content. Amorphous PLA is obtained at D-lactide content more than 15%. Moreover, the synthesized PLA from meso-lactide is also amorphous (Avinc and Khoddami 2009). On the other hand, pure PLLA is relatively stiff semi-crystalline material with ordered molecular structure and maximum Tm of 175–180 °C. Increasing the D-lactide level as an optical impurity in the PLA chain, at a constant molecular weight, leads to a decrease in the crystalline order, crystallization kinetics, Tm and Tg (See Fig. 2a). Furthermore, the desired hardness, toughness, biodegradation rate, melting temperature and crystallinity degree can be attained by adding D-lactide units to the PLLA chain. Normally, the D optical isomer as an impurity is present in the lactic acid obtained from the bacterial fermentation process (Avinc and Khoddami 2009). As it is evident in Fig. 2a, an augmentation in Kg increases the Tm and Tg of PLA, while these the molecular weight (Mn ) up to 10 mol Kg . temperatures remain constant by increasing the molecular weight more than 100 mol Kg Melting temperature of the PLA oligomer is 90 °C and for PLA with Mn of 100 mol reaches 185 °C (Saeidlou et al. 2012). Desirable properties of PLA such as transparency, high modulus, strength and rigidity, glossy appearance, good barrier properties and processability make it as an appropriate substitute for the petroleum-based plastics such as PET, PS, PP and high density polyethylene (HDPE) in various applications (Nofar et al. 2019; Siakeng et al. 2019a). Some of these uses are packaging applications, films, fibers and textiles, construction products and automobile parts (Nofar et al. 2019). PLA (Biomer L9000 grade) has properties such as Young modulus of about 3.6 GPa, tensile strength of about 70 MPa, strain at break of about 2.4%, flexural strength of about 98 mN2 , impact KJ strength of about 16.5 kJ, notched impact strength of about 3.3 m 2 , melt flow index of

Fig. 2 a Melting point versus D-lactic acid content in PLA chain, b Melting point and glass transition temperature versus number-average molecular weight. Adapted to (Bigg 2005; Hartmann 1998; Kolstad 1996; Saeidlou et al. 2012; Witzke 1997)

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Fig. 3 a Young and flexural modulus, b tensile and flexural strength, c elongation at break and d impact strength of PLA and some of common plastics g 3–6 10 min , density of about 1.25 cmg 3 and moisture absorption of about 0.3% (Hassan et al. 2013). Some properties of PLA are compared with the analogues properties of the highly consumed plastics in Fig. 3. It can be found in Fig. 3 that despite its biodegradability, PLA has comparable properties with common synthetic polymers, especially for Young modulus, flexural modulus and tensile strength. Furthermore, PLA can be processed and shaped to final products similar to other thermoplastic materials by using common melt processing techniques such as extrusion, injectionmolding, blow molding, thermoforming extrusion-coating and spinning (Hassan et al. 2013). PLA under shear flow field in the molten state follows the power-law like most of the petroleum-based polymers. For sheer rates up to 10 s−1 , it shows a Newtonian behavior, and for higher sheer rates, it demonstrates a shear-thinning melt behavior (Hamad et al. 2015). Pseudoplastic index of PLA melt is in the range of 0.2–0.3, depending on the chain structure. Moreover, entanglement threshold of PLA chains happens at molecular weights g . Molecular weight distribution and branching significantly affect of about 104 mol the melt viscosity of PLA (Avérous 2008). The zero-shear viscosity of PLA melt is roughly scaled the molecular weight with power of 3.4–4 (Ahmed and Varshney 2011). PLA dissolves in solvents such as chloroform, acetonitrile, dioxane, furan and fluorinated and chlorinated organic compounds. PLA’s non-solvents includes water, alcohols (such as methanol and ethanol) and alkanes (such as hexane and heptane) (Avérous 2008; Sharif et al. 2019). This aliphatic polyester is rather hydrophobic with mJ surface energy of 49 m 2 . PLA chains with L-lactide content of 92% and meso-lactide mJ level of 8% have dispersive surface energy (γ d ) of 37 m 2 and polar surface energy mJ p (γ ) of 11 m2 . The water absorption of PLA films is low similar to PET films. In packaging applications, the permeability of PLA films against CO2 and O2 gases is lower than PS and is in the range of PET permeability (Ahmed and Varshney 2011).

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In the biodegradation process, PLA chains are primitively degraded by hydrolysis reaction after being in moisture exposure for several months. The degradation process of PLA mainly consists of two steps: first, random and non-enzymatic chain scission of ester groups in abiotic conditions that results in a decrease in the molecular weight. Second step includes further molecular weight loss to reach the monomers and oligomers with low molecular weight. This process continues until the final mineralization under biotic degradation conditions. At this stage, the microorganisms can begin to metabolize the residues and produce CO2 and water (Savioli Lopes et al. 2012). Hence, the natural life cycle of biomasses is also possible for this biopolymer. Additionally, after serving the original purpose, it can be recycled and hydrolyzed to its building blocks, LA molecules (Hassan et al. 2013). Despite considerable advantages of PLA as a thermoplastic, which make it a competitive plastic with the common synthetic polymers such as PS, PE and PET, there are some disadvantages that limit the potential application range of PLA. Some of the major drawbacks of PLA are listed below: 1. Intrinsic brittleness: PLA fractures in very small strains, less than 10% (See Fig. 3). PLA is brittle even more than PS. The combination of brittle mechanical behavior with low impact strength confines the PLA applications (Hassan et al. 2013; Siakeng et al. 2019a). Different plasticizers such as lactide monomer, poly(ethylene glycol) (PEG) and poly(propylene glycol) (PPG) with low molecular weights and fatty acids are used to improve the brittle fracture behavior (Avérous 2008). 2. Slow biodegradation: The biodegradation rate of PLA depends on the crystallinity degree, molecular weight, molecular orientation, permeability, surface characteristics, chain stereochemistry and environmental conditions. In some cases, biodegradation of PLA took several years reportedly. The required long period of time for PLA biodegradation besides the high crystallinity degree of the residual fragments may cause problems. For instance, a second surgery was reported to be performed for removal a PLA-based implant after three years from the first surgery for implant embedment (Sharif et al. 2019). 3. Low dimensional stability: The T g of PLA is low (50–60 °C), comparing with the other polyesters. This temperature for PET is about 80 °C. This low temperature normally leads to low heat distortion temperature (HDT) and Vicat softening point. Hence, the service temperature for the PLA-based products are limited. For example, PLA is not suitable for packaging of foodstuffs containing hot liquids (Saeidlou et al. 2012; Sharif et al. 2019). 4. Slow crystallization rate: Due to the low crystallization rate of the biopolymer chains, the processability and service conditions of PLA have some limitations (Nofar et al. 2019). If the PLA chains can be crystallized to high extents, the corresponding HDT and Vicat softening point would shift to larger temperatures more than 30 °C and 100 °C, respectively (Saeidlou et al. 2012). 5. Low thermal stability: The rheological behavior of PLA melt is different from the behavior of common petroleum-based polymers and it has lower melt strength owing to the low chemical stability of PLA chains in the melt

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processing conditions. Consequently, PLA has narrower processing window (Siakeng et al. 2019a). Poor melt strength and low crystallization rate of PLA (as mentioned earlier) impose some restrictions on the processability, formability and foamability of the biopolymer (Nofar et al. 2019). 6. Hydrophobicity: The water contact angle of PLA is about 80°. Hence, the biological cells may have less tendency to adhere on the PLA surface (Sharif et al. 2019). 7. Throughout the literature, lots of researches have been devoted to find effective solutions for the mentioned disadvantages (Nofar et al. 2019). One relatively economical approach to improve the final properties of the PLA-based products is using the lignocellulosic fibers as reinforcement agents in the PLA matrix. This approach is expected to make a considerable improvement in the PLA performance, while maintains the renewable and biodegradable nature of the PLA-based products.

3 Lignocellulosic Fibers as Reinforcement Agents Fillers are used in polymers to improve the properties and decrease the cost of final products. The common and conventional fillers of polymers are glass and carbon fibers, kevlar, silica, calcium carbonate, clay, graphite, carbon black and carbon nanotubes (Zhang et al. 2017). Using synthetic fibers as reinforcement agents in plastic matrices gives the composites the desired durability and structural properties required in many applications. However, the energy crisis, depletion of petroleum resources and growing environmental concerns have led to a growing demand for the renewable resources (Sisti et al. 2018). Unlike man-made fibers, natural fibers as renewable resources do not cause environmental issues in their manufacturing process and disposal. Moreover, natural fibers have some advantages such as light weight, low cost, renewability, biodegradability, recyclability, flexibility, ecologically friendly, versatility and high specific strength (Zhang et al. 2017; Zimniewska and Wladyka-Przybylak 2016). Based on the used resources, natural fibers are categorized to three distinct groups, including animal, mineral and plant fibers. The animal fibers such as wool, silk and hair are used for reinforcing purposes have proteins as their major component. The mineral fibers like asbestos have been also applied to reinforce polymers. However, their applications have carcinogenic effects by inhalation and ingestion. Plant fibers, which are lignocellulosic materials, contain cellulose as a main constituent. It has been repeatedly reported that the plant fibers have higher strength and stiffness comparing to the animal fibers. Therefore, plant fibers are the most appropriate reinforcement agents for polymer matrices among all natural fibers (Mochane et al. 2019; Zhang et al. 2017). Plant fibers have gotten a special place as reinforcement agents in the composite market in recent decades and today, the plant fiber-based biocomposites are extensively used in different industries from automotive to construction sectors (Sisti et al.

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2018). There is a wide range of fibrous plants, more than 2000 species, which can be applied as reinforcement agents in polymers. However, only plants having high fiber contents have been applied on industrial scale, owing to the possibility for easy and effective fiber extraction (Zimniewska and Wladyka-Przybylak 2016). The most important fibers used in polymer-based composites are flax, hemp, jute, ramie, kenaf, sisal, coir, abaca and pineapple leaf fibers (Siakeng et al. 2019a; Sisti et al. 2018; Zimniewska and Wladyka-Przybylak 2016). Based on the utilization routes, the plants used in the natural fiber fabrication process are classified into primary and secondary plants. Primary plants are those that are cultivated just for fiber production such as cotton, jute, java-kapok, hemp and kenaf. In contrast, secondary plants are the ones from which fibers are produced as by-products like banana, coconut coir, pineapple, oil palm and rice straw (Siakeng et al. 2019a). The lignocellulosic fibers have enough potential to replace the synthetic fibers such as glass fibers in reinforcing polymers. In addition to their renewability and biodegradability, natural fibers have been considered as beneficial reinforcement agents because of their favorable properties such as low density and appropriate mechanical performance. The natural fibers with a density of 1.15–1.5 cmg 3 are much lighter than E-glass fibers with a density of 2.4 cmg 3 . The usage of plant fibers, as renewable resources, leads to the fabrication of “green” products, which are recyclable on industrial scale. As a result, this eventuates in less emission of greenhouse gases and reduced generation of pollutant wastes. For example, the cultivation of hemp in an agricultural land with area of 1 ha leads to the absorption of approximately 3.5 tons of atmospheric carbon dioxide gas in one harvest season. On the other hand, the production of 1 ton of polypropylene (PP) synthetic plastic release 3 tons of CO2 gas into the atmosphere (Zimniewska and Wladyka-Przybylak 2016). Moreover, plant fibers like flax, hemp and ramie fibers have relatively good mechanical properties and their specific modulus and strength are comparable to the synthetic glass fibers (Kozlowski and Wladyka-Przybylak 2004). In Fig. 4, the physical appearance and some of the physical and mechanical properties of lignocellulosic fibers and synthetic fibers are compared with each other. It is clear from this figure that the elongation at break for natural fibers are often higher than the ones for glass and carbon fibers, hence resulting in a better performance of natural fiber-reinforced composites. Furthermore, plant fibers are suitable for the production of thermal insulators, because of the low thermal conductivity W (Kozlowski and Wladyka-Przybylak of natural fibers in the range of 0.29–0.32 m.K 2004). In general, the advantages of lignocellulosic fibers in polymer-based composites over the synthetic fibers can be briefly mentioned as follows: Sustainability and renewability, easy accessibility and lower cost, biodegradability, thermal recyclability, acceptable specific strength and other specific properties, easy extraction of fibers, low density, good thermal properties, more friendly processing and less tool wear, no skin irritation and improved energy recovery. Their main disadvantages can be outlined as follows: Lower strength specially impact strength, higher moisture absorption and fiber swelling, limited maximum processing temperatures, poor flame resistance, variable quality (The properties of

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Fig. 4 Physical appearance of plants, the resultant natural fibers as well as synthetic fibers, (a)– (e) column charts for some of the physical and mechanical properties of plant and synthetic fibers (Ballesteros et al. 2018; Mochane et al. 2019; Siakeng et al. 2019a)

plant fibers are strongly dependent to the crop variety, ecotype, maturity, cultivation place, retting process and fiber processing tools.), less durability (Interfacial adhesion of polymer matrix and fibers must be modified) and price fluctuations. Despite some drawbacks in properties, plant fibers have been generally accepted as the most promising alternative of synthetic fibers in many polymeric composite applications. All plant fibers are naturally lignocellulosic and comprise of cellulose, hemicellulose, lignin, pectin and waxes, except for cotton fibers. In fact, more than 80% of dried weight of lignocellulosic fibers include cellulose, hemicellulose and lignin.

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The small remaining part constitutes of other fiber components such as pectin, waxes and protein (Shafiei et al. 2015; Zhang et al. 2017). The cellulose content of each fiber determines the fiber strength and modulus. Most of the natural fiber properties including the mechanical characteristics depend on the cellulose type, microfiber angle, degree of polymerization and overall cellulose content of the fiber (Siakeng et al. 2019a). The composition of various fiber components is controlled by the nature of fibers and the soil and environmental conditions, in which the plant is grown. The chemical compositions of some plant fibers are shown in Table 1 the lignocellulosic fibers are categorized into seven groups based on their origins as: 1. Fibers from bast/stem including jute, flax, hemp, ramie, mesta, kenaf and roselle 2. Fibers from fruit such as coir and oil-palm 3. Fibers from grass/reeds like bamboo, bagasse, corn, sabai, rape, elephant grass, esparto and canary 4. Fibers from leaf including pineapple, sisal, banana, abaca, henequen, agave and raphia 5. Fibers from seed containing cotton, kapok, milkweed and loofah 6. Fibers from stalk like wheat, maize, barely, oat, rice and rye 7. Fibers from wood consisting of hardwood and softwood (Kozlowski and Wladyka-Przybylak 2004; Siakeng et al. 2019a; Sisti et al. 2018). The bast fibers as reinforcement agents are embedded in the outer layer of bast and encompass the plant stem, thereby stabilizing the plant. With the highest cellulose contents, the bast fibers provide effectual reinforcement effect in the biocomposites. It has been reported that the leaf fibers improve the composite toughness. Moreover, the seed and fruit fibers reportedly eventuate in the elastomeric behavior of the composites (Sisti et al. 2018). Differences in the chemical compositions and physical characteristics results in a wide range of properties for the plant fibers (Kozlowski and Wladyka-Przybylak 2004). It can be seen in Table 1 that carbohydrates along with lignin are the main components of the lignocellulosic materials. The major components of the plant cell wall are cellulose and hemicellulose carbohydrates (Shafiei et al. 2015). Pectin in lignocellulosic fibers acts like an adhesive and holds the fibers in bundles in the non-fibrous tissues. In lignocellulosic materials, hemicellulose, pectin and lignin play the role of matrix, wherein cellulose fibers are localized as a reinforcement agent (Sisti et al. 2018). In the following part, different components of plant fibers are briefly described.

3.1 Cellulose As the main component of all plant fibers, cellulose is a linear polymer, which is composed of D-anhydroglucose units. These units are interlinked to each other at carbon-1 and carbon-4 atoms through β-1,4-glucosidic bonds (Saha et al. 2016).

68.6–76.2

Ramie

41–57

38–45

Wheat

65

Sisal

Rice

70–80

Pineapple

Stalk

77.6

Henequen

83–91

60–65

Cotton

56–63

45.6

Elephant grass

Banana

73.83

Bamboo

Abaca

55.2

Bagasse

Seed

Leaf

Grass/reeds

65

45–57

Kenaf

Oil-palm

61–71

Jute

32–43

68

Hemp

Coir

71

Flax

Bast

Fruit

Cellulose (wt%)

Fiber type

15–31

33

3

12

18.8

4–8

6–8

20–25

–

12.49

16.8

–

0.15–0.25

13–16

21.5

14–20

15

18.6–20.6

Hemi-Cellulose (wt%)

12–20

8–19

–

9.9

12.7

13.1

5–10

7–9

17.7

10.15

25.3

29

40–45

0.6–0.7

8-13

12–13

10

2.2

Lignin (wt%)

–

–

0.6

10

1.1–1.2

–

–

–

–

0.37

–

–

3–4

1.9

3–5

–

1

2.3

Pectin (wt%)

–

8–35

8–9

2

3.2–4.2

–

–

3

–

–

–

–

–

0.3

–

0.5

0.8

1.7

Wax (wt%)

Table 1 Chemical compositions of some common lignocellulosic fibers (Mochane et al. 2019; Siakeng et al. 2019a)

6.8

14–20

–

0.6–1

0.9–1.2

–

9.6

3

5

–

1.5–5

2.4

2.7–10.2

–

2–5

0.8

0.8

–

Ash (wt%)

–

–

20–30

10–22

8–15

–

11

20–25

–

–

–

46

30–39

7.5

2–6.2

8

2–6.2

5–10

Microfibril angle (degree)

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In cellulose chain, two adjacent glucose molecules named as cellobiose rotates 180 degrees around the chain backbone, relative to the neighboring repeat unit (See Fig. 5a) (Shafiei et al. 2015). Each glucose unit has three hydroxyl groups that can make inter- and intra-molecular hydrogen bonds. Some of these bonds are illustratively shown in Fig. 5b. Therefore, the plant fibers are intrinsically hydrophilic. The inter-molecular O3 –HO5 hydrogen bonding gives the cellulose a linear and hard structure (Kozlowski and Wladyka-Przybylak 2004). Furthermore, hydrogen bonds cause the cellulose chains to mostly orient in a parallel and ordered structure. The cellulose chains with ordered orientation form the crystalline cellulose owing to lack of side groups. The crystalline cellulose plays the main role in the fiber tensile strength and flexibility (Saha et al. 2016). Cellulose may have crystalline, para-crystalline or amorphous structures depending on the source and cellulose extraction method (Shafiei et al. 2015).

Fig. 5 a Cellulose repeating unit, b intra- and inter-molecular hydrogen bonds in cellulose respectively shown by blue and red colors and c main monomeric units of lignin (Orue et al. 2018; Shafiei et al. 2015)

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3.2 Hemicellulose Hemicelluloses are hetero polymers formed from polysaccharides and polyuronides, which are synthesized and packed in the Golgi vesicles of the plants. Various types of hexosans (mannan, glucosan, galactan and rhamnan) and pentosans (arabinan and xylan) are available in the hemicellulose polysaccharides (Shafiei et al. 2015). Hemicellulose chemically differs from cellulose in several aspects: 1. The hemicellulose macromolecule consists of different sugar units. 2. The degree of polymerization of hemicellulose is 10 to 100 times lower than that of cellulose. 3. Hemicellulose has amorphous structure owing to many chain branches along the polymer backbone. 4. Hemicellulose is more hydrophilic than cellulose and is highly susceptible to acid or alkali hydrolysis. The removal of hemicelluloses from the lignocellulosic fibers increases the tensile strength of fibers (Saha et al. 2016).

3.3 Lignin Lignin is a complex and crosslinked polymer having an amorphous and 3dimensional network structure. Different aliphatic and aromatic monomers like p-coumaryl alcohol, P-coniferyl alcohol and sinapyl alcohol compose the lignin networks (Fig. 5). Moreover, the subunits are present in lignin structure with side groups (Saha et al. 2016; Shafiei et al. 2015). The cement like behavior of lignin leads to an improvement in the strength of crystalline cellulose in the layers of plant cell walls, hence resulting in plant growth to reach higher heights. On the other hand, the hydrophobicity of lignin facilitates the water and nutrient transfer (Shafiei et al. 2015). Moreover, lignin protects the plant well, against microbial attacks, moisture and external environmental factors (A. Orue et al. 2018).

3.4 Other Components As mentioned earlier, more than 80% of dried weight of lignocellulosic materials is composed of cellulose, hemicellulose and lignin. The residual part includes many different materials in small quantities. These materials can be divided into the extractive and non-extractive substances. The first group, which are extracted by polar and non-polar solvents, comprises of fats, fatty acids, isoprene, alcohols, ketones and phenols. The latter group consists of proteins, pectin, alkali-earth carbonates, starch, silica and oxalates (Shafiei et al. 2015). The quality and properties of lignocellulosic fibers have a great importance in the polymer-based composite performance. The main disadvantage of natural fibers

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compared with synthetic fibers is that the natural fibers do not have a constant composition and quality. The reasons behind this go to the climate conditions, crop variety and retting process. For instance, low temperatures and high relative humidity of growing season bring about the formation of more delicate and longer fibers. Besides, the efficiency of retting process, in which pectin is broken down and the fibers are released, considerably affects the fiber quality (Sisti et al. 2018). Nonetheless, the lignocellulosic fibers are applied in different polymer matrices owing to the unique and favorable properties. The plant fiber-reinforced polymer composites are classified into different groups based on the applications. One of them is the common panel composite, wherein the lignocellulosic material is the main part, such as fiberboards and insulation boards. Polymers are used as binding materials in these applications. Another group of the reinforced composites is attributed to the products in which the lignocellulosic materials as reinforcing fillers are dispersed in a polymer matrix including rubbers, thermoplastics and thermosets. The other group is related to the nonwoven textile composites (Kozlowski and Wladyka-Przybylak 2004). Different polymers are employed in producing lignocellulosic composites including phenolic resins, epoxy resins, polyesters, polyimides, polypropylene (PP), polyurethanes (PU), polyamides (PA), polyethylene (PE), polystyrene (PS), poly(vinyl chloride) (PVC), india-rubber, starch, PLA, cellulose esters, tannin and poly(hydroxy butyric acid) (PHB) (Kozlowski and Helwig 1998; Kozlowski and Wladyka-Przybylak 2004). Mechanical properties of the composites reinforced by lignocellulosic fibers are considerably affected by various factors such as fiber type, fiber dispersion state in matrix phase, fiber orientation and matrix/filler interfacial adhesion. The fibers having large cellulose content, in which the cellulose micro-fibrils are oriented along the fiber axial direction, show the highest mechanical performance including ramie, hemp and flax (Mochane et al. 2019). However, one may keep in mind that the low interfacial strength of most polymers with lignocellulosic fibers prevents them from competing with the reinforced composites containing synthetic fibers such as glass, carbon and aramid fibers (Kozlowski and Helwig 1998). In fact, the plant fibers have not enough compatibility with most polymers, due to the significant difference in polarity and surface energy. The matrix/filler incompatibility provokes the poor interactions between different components and formation of weak interface (Kozlowski and Wladyka-Przybylak 2004). To overcome the weakness and subsequent consequences, various treatment methods for natural fibers have been developed which are discussed in detail in the following sections. However, the development of natural fiber-reinforced composites is progressing, and in this sector, a special attention is paid to the biodegradable polymer-based composites reinforced by plant fibers. Biodegradability and sustainability give these green composites a unique opportunity to be introduced as the most promising substitutes for the conventional polymer composites. Among all the biodegradable polymers, PLA is the most frequently used polymer in plant fiber-reinforced composites (Zhang et al. 2017).

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4 Lignocellulosic Fiber-Reinforced PLA Composites: Pros and Cons As mentioned earlier, the plant fibers-reinforced biodegradable polymers are the best candidates for “green composite” production (Mochane et al. 2019). The plant fiber-reinforced PLA biocomposites have received lots of attentions in recent years (Mochane et al. 2019; Siakeng et al. 2019a). Using PLA/plant fibers composites result in less dependence on the exhaustible resources of fossil fuels and less environmental pollution owing to the special characteristics such as eco-friendly, biodegradability, sustainability and renewability (Siakeng et al. 2019a). As it is emphasized in section one, despite some desirable properties, PLA requires to be reinforced by fibers or fillers to improve the performance. The lignocellulosic fiber-reinforced PLA composites have some favorable properties such as high stiffness and specific strength, good processability, recyclability and biodegradability (Siakeng et al. 2019a). However, one may notice that the industrial applications of PLA is limited due to the low availability and high production cost. The incorporation of abundant plant fibers into the biopolymer is in fact an approach to develop sustainable PLA products (Abu Ghalia and Dahman 2017). In order to gain a noticeable market share of plastics, the PLA biocomposites must present properties comparable to the synthetic polymers (Siakeng et al. 2019a). Basically, the performance of the PLA/natural fiber biocomposites depends on different factors including physical and mechanical properties of the components (matrix and reinforcing fibers), matrix/fiber interactions, and orientation of reinforcing fibers (Zhang et al. 2017). The most important weaknesses of PLA/lignocellulosic fiber composites are the non-uniform properties of the fibers and degree of polymerization, poor matrix/fiber adhesion, high flammability, and moisture sensitivity (Siakeng et al. 2019a). The hydroxyl groups along with other functional groups in the lignocellulosic material makes the plant fibers extremely hydrophilic (Mochane et al. 2019). In other words, the fibers are highly hydrophilic because the plentiful hydroxyl groups in the chemical structure favor the formation of strong intra- and inter-molecular hydrogen bonding (Kozlowski and Wladyka-Przybylak 2004). The strong hydrophilicity of fibers consequently leads to the most important drawback of plant fibers as reinforcement agents, i.e. poor interfacial adhesion between hydrophilic lignocellulosic fibers and hydrophobic polymer matrix (Siakeng et al. 2019a). One may know that matrix/fiber adhesion is determined by the interfacial forces including primary and secondary ones. The primary forces are short range ones that originate from the ionic, metallic and covalent bonds. The secondary or intermolecular forces are provoked by physical attractions such as Van der Waals interactions (Orue et al. 2018). Because of the large differences in polarity and surface energies, weak interfacial forces form between natural fibers and PLA matrix. Consequently, lignocellulosic fibers in PLA matrix have very poor compatibility and wettabililty. The resultant weak interface between the dispersed and matrix phases arises a substantial reduction in the interfacial adhesion and strength. The interfacial strength plays a key role in determining the final properties of the PLA-based biocomposites. The effective transfer of load from

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the matrix to the reinforcing fibers has a prerequisite, i.e. adequate matrix/fiber interfacial adhesion. Once the prerequisite is met, the desirable mechanical properties of the biocomposite is attainable (Mochane et al. 2019; Zhang et al. 2017). Generally, the PLA-based biocomposites containing untreated natural fibers show extensive fiber pull-out, regardless of the plant fiber type used in PLA (Huber and Müssig 2008; Mochane et al. 2019). Moreover, weak bonding strength between the reinforcing phase and PLA chains results in matrix breakage and matrix/fiber debonding under stress and mechanical deformations. The scanning electron microscopy (SEM) images of the fractured surfaces of PLA/lignocellulosic fiber biocomposites in Fig. 6 obviously show poor fiber wetting by PLA matrix phase, large-scale debonding and fiber pull-out.

Fig. 6 SEM image of fractured surface of: a PLA/flax fiber composite (Liber-Kne´c et al. 2015), b PLA/coir fiber composite (Sun et al. 2017), c and d PLA/cellulose fiber composite (Courgneau et al. 2013), e PLA/rice straw fiber composite (Yaacob et al. 2016) and f PLA/flax fiber composite (Aliotta et al. 2019)

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Additionally, the hydrophilic nature of plant fibers eventuates in high water absorption of the biocomposites. The absorbed water content of fibers depends on the relative humidity of the ambient atmosphere, fiber purity and cellulose crystallinity degree. All hydroxyl groups in the amorphous regions of the lignocellulosic fiber structure are accessible sites for water absorption unlike the –OH groups inside the crystalline phase. The biocomposite strength deteriorates by water absorption. Furthermore, the high hydrophilic nature of plant fibers creates another challenge, which encounters in the fiber/matrix mixing process. The challenge attributes to the attainment of a uniform dispersion and distribution state of fibers and prevention of the formation of fiber aggregates in matrix (Kozlowski and Wladyka-Przybylak 2004). Undoubtedly, without uniform dispersion and distribution of reinforcing fibers in matrix, the biocomposite could not show an optimal performance. By improving the fiber/matrix interfacial adhesion, the dispersion state of fibers improves and less voids around the fibers embedded in the matrix are formed. Clearly, the processing parameters such as temperature, pressure and stress flow fields can be properly adjusted to improve the fiber dispersion state. For instance, intense extensional flow fields in twin-screw extruders can facilitate the formation of more uniform mixture with better dispersion state of the reinforcing fibers. However, this melt-compounding process damage the fibers more considerably, comparing with the process using single-screw extruders. Consequently, the fiber length along with the aspect ratio decrease to higher extents. On the other hand, the mechanical properties of the biocomposite can be improved by adding more fibers to the PLA matrix. Nonetheless, there is an optimum value for the fiber loading. In larger fiber contents, the processability of the composite melt is worsened and more voids and lumens are formed and trapped around the fiber bundles. Consequently, this phenomenon restrains the fiber/matrix melt compaction, hence causing poor mechanical properties such as strength (Mochane et al. 2019). Reduced thermal stability is another notable drawback of the PLA/lignocellulosic fiber composites comparing with the virgin PLA (Siakeng et al. 2019a). In a study (Yussuf et al. 2010), PLA/rice husk and PLA/kenaf fiber biocomposites, without fiber treatment, showed less thermal stability in comparison with the pure matrix. However, PLA/kenaf fiber composites have higher thermal stability relative to the PLA/rice husk composites owing to different compositions of the filler constituents. Moreover, it has been frequently reported that the tensile strength, strain at break, flexural strength and impact strength of PLA matrix diminish to lower values by adding plant fibers due to the weak interfacial adhesion (Faludi et al. 2013a; Nyambo et al. 2011; Sisti et al. 2018). A brief review on the PLA/untreated plant fiber composites is presented in Table 2. As explained so far, it is obvious that the treatment of lignocellulosic fibers is necessary to improve the performance of the fiber-reinforced PLA biocomposites. Through the literature, researches repeatedly confirm that the surface treatment of plant fibers, as an efficient and effective approach, results in improved matrix/fiber interfacial adhesion, better mechanical performance and lower moisture sensitivity (Siakeng et al. 2019a; Zhang et al. 2017). The fiber treatment methods can be categorized into chemical, physical, physico-chemical and biological modifications (Siakeng et al. 2019a). By applying these methods, the fiber impurities are

30 wt%

Wheat straw

Compounding by twin-screw extruder/Injection molding

Compounding by twin-screw extruder/Injection molding

10–40 wt%

Wheat straw, corn stover and soy stalks

Compounding by internal mixer/Hot press

Processing

Hot press

1–30 wt%

Hemp fibers

Cotton, flax and hemp –

Fiber content

Fiber type Masirek et al. (2007)

Authors (year)

• Increases in the Young modulus, flexural modulus and water absorption • A decrease in the flexural and tensile strength

• A decrease in the PLA crystallization temperature, T c • An increase in the flexural and tensile modulus • A decrease in the flexural, tensile and impact strength • More brittle failure mode of PLA • Matrix/fiber debonding, fiber pull-out and fiber fracture are observed on the SEM images of the composite fractured surfaces

(continued)

Nyambo et al. (2010)

Nyambo et al. (2010)

• The interfacial shear strength of composites Huber and Müssig (2008) was determined by the single fiber fragmentation test. • Poor interfacial adhesion between PLA matrix and fibers results in insufficient stress transfer to break the fibers.

• An increment in the elastic modulus up to 5.2 GPa at 20 wt% loading of hemp fibers by increasing the filler content • A decrease in the stress and strain at break by increasing the fiber content

Observed results after adding fibers to PLA

Table 2 Some researches on the unmodified lignocellulosic fiber/PLA biocomposites

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Compounding by internal mixer/Compression molding

Compounding by twin-screw extruder

Wood, 5–60 vol% micro-crystalline cellulose and corn cob

Triticale straw

10–40 vol%

Compounding by internal mixer/Compression molding

0–60 vol%

Wood

Compounding by internal mixer/Compression molding

5–60 vol%

Corn cob

Processing

Fiber content

Fiber type

Table 2 (continued) Faludi et al. (2013a)

Authors (year)

Faludi et al. (2014)

• A decrease in the PLA thermal stability and Mihai and Ton-That PLA HDT (2014) • A decrease in the impact strength by increasing the fiber loading • A decrease in the tensile strength with fiber loading of 10 and 20 vol% in contrast to the tensile modulus, flexural modulus and flexural strength (continued)

• A decrease in the PLA tensile strength for different loadings of fibers • The occurrence of fiber breakage in microscopic observations • Reported the need for using natural fibers with higher strength to prepare the biocomposites having improved properties

• An increase in the Young modulus and a Faludi et al. (2013b) decrease in the tensile strength • The addition of more fibers to PLA leads to higher increment in the Young modulus and larger decrease in the tensile strength.

• An increase in the Young modulus • A decrease in the tensile strength • Further increase in the fiber content results in more increment in the Young modulus and more decrease in the tensile strength. • Matrix/fiber debonding, fiber pull-out and matrix local yielding are observed on the fractured surface

Observed results after adding fibers to PLA

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Fiber content

10 wt%

5–20 wt%

5–10 wt%

5–15 vol%

30 wt%

10–30 wt%

Fiber type

Chopped flax fibers

Paddy straw powder

Açaí short fibers

Rice straw

Wood, bleached kraft pulp and flax fiber

Sisal fibers

Table 2 (continued) Processing

Compounding by internal mixer/Compression molding

Compounding by twin-screw extruder/Injection molding

Sequential layering and batch-mixed using hot press

Cast molding

Compounding by internal mixer/Compression molding

Compounding by twin-screw extruder/Injection molding

• An increase in the tensile modulus • Decreases in the tensile strength and strain at break • Decreases in T g , T m and crystallinity degree of PLA at fiber loading of 30 wt%

• No significant improvement in the tensile strength in contrast to the elastic modulus by adding medium density fiberboard (MDF) fibers • A decrease in the notched impact strength • Microscopic observations of severe fiber pull-out on the fractured surface

• A decrease in the flexural strength by increasing the fiber content in PLA biocomposites

• No significant improvement in the flexural modulus by adding fibers to PLA

• A decrease in the tensile strength and elongation in break due to poor fiber wetting by PLA matrix • Improved biodegradability of PLA in compost

• No significant improvement in the average fatigue strength by adding short flax fibers

Observed results after adding fibers to PLA

(continued)

Lagazzo et al. (2019)

Thumm et al. (2018)

Augustine Saidi et al. (2017)

Santos et al. (2017)

(Yaacob et al. 2016)

Liber-Kne´c et al. (2015)

Authors (year)

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Fiber content

30 wt%

Fiber type

Pineapple fibers and coir fibers

Table 2 (continued) Compounding by internal mixer/Hot press

Processing • An increase in the tensile and flexural modulus • Microscopic observations of fiber breakage and pull-out on the fractured surface of the coir fiber-reinforced composites along with weak fiber distribution and some voids on the surface • No voids on the surface of the pineapple fiber-reinforced composites are observed indicating better matrix/fiber interactions

Observed results after adding fibers to PLA Siakeng et al. (2019b)

Authors (year)

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eliminated, fiber hydrophilicity is reduced and stronger bonding or adhesion between matrix and reinforcing fibers is formed (Kozlowski and Wladyka-Przybylak 2004). The following section is devoted to different fiber modification methods applied in the PLA/natural fiber biocomposites.

5 Fiber Treatment Methods 5.1 Alkali Treatment Alkali treatment that is also known as mercerization is one of the most efficient chemical treatment methods, which is widely applied for the surface treatment of plant fibers (Zhang et al. 2017). This simple and effectual method improves the matrix/lignocellulosic fiber adhesion (Orue et al. 2018). One may know that the shortterm and long-term compatibility of fibers with polymer matrix mainly depends on the successful removal of the non-cellulosic components of the fiber surface like waxes, pectins, hemicelluloses and lignins. In fact, these impurities hinder matrix/fiber interfacial bonding and makes the biocomposites sensitive to moisture and UV irradiations. The most feasible technique to confront the problem is alkali treatment (Islam et al. 2010). The main performance of the mercerization method to improve the matrix/fiber adhesion is important from two different aspects: 1. During the mercerization process, the fiber bundles break and fibrillization happens. By the formation of shorter fibers with smaller diameters, the surface area of the reinforcing fibers increases. Therefore, the larger area of matrix/filler interfacial surface is provided for transferring stress from matrix to the reinforcing fibers (Siakeng et al. 2019a; Zhang et al. 2017). The alkali treatment reaction is usually performed by soaking the plant fibers in sodium hydroxide aqueous solution at a predetermined temperature for a particular period of time. Throughout the process, Na+ ions diffuse into the crystalline cellulose and cause fiber swelling. As a result, the hydroxyl groups on the fiber surface are ionized into alkoxides, which is schematically drawn in Fig. 7. The ionization reaction

Fig. 7 Proposed alkali treatment reaction of lignocellulosic fibers

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in the crystalline regions converts the parent cellulose (cellulose I) to the alkaline cellulose. By washing the treated fibers with water, which subsequently performs after the ionization reaction, the crystalline cellulose regenerates and transforms to the cellulose II having more thermodynamic stability (Aydın et al. 2011; Islam et al. 2010; Kozlowski and Wladyka-Przybylak 2004; Zhang et al. 2017). This transformation is an exothermic and irreversible process (Kozlowski and Wladyka-Przybylak 2004). 2. Through the surface treatment, the non-cellulosic components such as hemicellulose, lignin, waxes and other organic materials are removed from the fiber surface. The removal of natural impurities of fibers occurs alongside the removal of artificial impurities, if any exist (Zhang et al. 2017). Since hydroxyl groups are covered with non-cellulosic substances, the removal of non-cellulosic components thus provides more hydroxyl bonding sites on the fiber surface for more effective adhesion to the matrix (Islam et al. 2010). Alkaline treatment, based on the mentioned phenomena, results in higher surface roughness. Accordingly, the surface area of plant fibers is enlarged and the mechanical interlocking of fibers with matrix is improved, thereby leading to an enhancement in the interfacial adhesion and compatibility of the fibers and matrix. The mercerization method is applied as a pretreatment method in many modification processes such as silanization, esterification and etherification to increase the cellulose content on the fiber surface and provide more sites for the subsequent treatments (Le Moigne et al. 2014; Way et al. 2012; Zandi et al. 2019). The alkali method also leads to a noticeable augmentation in the amorphous cellulose level (Zhang et al. 2017). The effects of alkali treatment on various properties of the lignocellulosic fibers are controlled by the treatment process conditions such as the NaOH solution concentration, temperature and duration of the reaction. It is noteworthy that by the removal of non-cellulosic components, some voids can be formed in the fiber structures throughout the process. Moreover, the reactions can depolymerize the native cellulose and collapse the crystalline structures of cellulose (Shafiei et al. 2015). By that, the alkali treatment can lead to a severe damage to the fiber structure and weaken the mechanical properties. Thus, the treatment process must be carried out by using the predetermined optimal conditions of the alkali reaction for each type of the plant fibers. The SEM images of Fig. 8 show the structural changes in the short flax fibers against the NaOH concentration in the aqueous solution over the mercerization process. Taking a look reveals that the fiber bundles break more extensively and fibrillation occurs by increasing the NaOH concentration. Additionally, a decrease in the fiber diameter and an increment in the fiber aspect ratio can be observed after removal of the natural and artificial impurities. Besides, a rougher topography of the fiber surface is obtained after treatment (Aydın et al. 2011). A literature review on the PLA/lignocellulosic fiber biocomposites show that the alkali treatment method is applied for different types of plant fibers to improve the matrix/filler interfacial bonding (Jandas et al. 2012; Le Moigne et al. 2014; Ander Orue et al. 2016; Saidi et al. 2018; Way et al. 2011, 2012; Zafar et al. 2016). However, different changes in the properties of PLA/lignocellulosic fiber

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Fig. 8 a and b Untreated flax fibers, c and d treated flax fibers with 10% NaOH solution, e and f treated flax fibers with 30% NaOH solution. Reproduced with permission from (Aydın et al. 2011)

biocomposites have been reported after the mercerization treatment, demonstrating the considerable importance of the treatment process conditions for tailoring the properties of the modified fibers as well as the interfacial phenomena of PLA matrix and lignocellulosic fibers. In one of these researches (Huda et al. 2008), an effort was made to improve the mechanical and thermal-mechanical properties of laminated PLA/fiber biocomposites by the mercerization treatment of pineapple leaf fibers (PALF). The laminated

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biocomposites were prepared by compression molding process through film stacking method. The alkali treatment on PALF was carried out by using 5% NaOH (w/v) solution. The fractured surface of the biocomposites containing untreated PALF was revealed poor distribution and dispersion states of the fibers. Large voids and multifiber bundles were also observed on the surface. The fracture was accompanied by severe fiber pull-out and fiber surfaces were completely clean with no stacked residual matrix parts on them indicating poor interactions between PALF and PLA chains. Despite that, the alkali treatment of fibers and the resultant increment in the fiber roughness brought about stronger fiber/matrix interactions and mechanical interlocking. By treating the fibers, the impact strength of PLA matrix increased up to 86% by incorporating modified PALF. Furthermore, the applied alkali treatment led to an increase in the flexural modulus and strength for PALF/PLA biocomposites. The findings of the research have disclose that, at a constant fiber loading, the impact strength and flexural modulus of the biocomposites having alkali treated fibers are higher than even the related data of the biocomposites containing silane-treated fibers (Huda et al. 2008). In another work (Islam et al. 2010), the PLA/fiber biocomposites having short and long hemp fibers were obtained using compression molding process and film stacking method. In fact, some preforms including short and long fiber mates from both treated and untreated fibers were prepared and applied to shape the biocomposites along with the PLA sheets. The surface treatment of the fibers were done by using NaOH and Na2 SO3 solutions. In pull-out test, the interfacial shear strength (IFSS) of the treated fibers improved comparing to the untreated fibers. Moreover, the fiber heterogeneous nucleation effect on the PLA crystallization process is intensified by the surface treatment and the corresponding better fiber/PLA interactions. Matrix/fiber debonding, fiber pull-out and holes at the fractured surface of the biocomposite containing treated hemp fibers are observed to a lesser extent. The researchers additionally showed that the alkali treatment leads to higher fracture toughness, tensile modulus, flexural modulus and strength. However, the impact strength of the biocomposites containing the treated fibers is lower than the untreated fiber-reinforced PLA composites. The most probable reason was supposed the occurrence of less fiber pull-out and delamination during crack initiation and propagation in the treated fiber/PLA composite, which leads to a reduction in the dissipated energy (Islam et al. 2010). The effect of reaction temperature and NaOH solution concentration of alkali modification process on the tensile and flexural properties of wheat straw fiber/PLA biocomposites were studied later by another research group (Pan et al. 2012). The optimum mechanical properties of the mentioned biocomposites were obtained at sodium hydroxide concentration of 2.5% and reaction temperature of 125 °C. At higher reaction temperatures and larger sodium hydroxide concentrations, more hemicellulose and lignin are removed from the fiber surfaces and the fiber microstructure changes more, hence deteriorating the mechanical properties of fibers. However, one may keep in mind that the lower concentrations of NaOH solution in the treatment reaction is more eco-friendly (Pan et al. 2012). To improve the agricultural waste utilization, skin fibers of durian fruit as a lignocellulosic reinforcement agent were also used for the PLA matrix (Manshor et al.

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2014). Durian skin fibers (DSF) are among short lignocellulosic fibers. Poor interface of the untreated fibers with PLA matrix and stress concentration at the end of these fibers result in the formation of microcracks through crack propagation, thereby decreasing the impact resistance of PLA matrix. The fiber surface treatment with NaOH solution has led to an improvement in the impact strength, flexural strength, flexural modulus and thermal stability of the DSF/PLA biocomposites (Manshor et al. 2014). Besides sodium hydroxide alkali solution, researchers (Siakeng et al. 2018) also applied calcium hydroxide (Ca(OH)2 ) solution for the chemical treatment of coir fibers (CF) and PALF in the PLA-based biocomposites. It is shown in this research that the fiber diameter of CF and PALF after NaOH treatment change from 220.66 and 79.4 μm to 156.33 and 50.6 μm, respectively. Similarly, the use of Ca(OH)2 treatment induces a reduction in fiber diameter form the mentioned initial ones to 158.66 and 62.78 μm, respectively. Figure 9 shows the SEM images of CF and PALF after NaOH, Ca(OH)2 and silane treatments in this work. As can be observed, the surface of untreated fibers are covered with impurities. By the chemical treatment, the impurities such as waxes are removed from the surfaces, which makes the fiber surfaces cleaner. The CF surfaces after the treatments, especially with NaOH, have many nodes and pinholes, which assists the mechanical bonding of fibers with matrix chains. The effects of Ca(OH)2 treatment on the biocomposite performance were observed to be less pronounced comparing with those of the other employed treatments in this study, i.e. NaOH and silane. Since Ca(OH)2 modification reaction relatively has lower yield in changing the component compositions on the fiber surface, the Ca(OH)2 treated fibers have weaker reinforcing performance and decrease the strength and strain at break of the untreated fiber-reinforced PLA biocomposites. However, the NaOH treatment comparatively causes the strongest interfacial bonding between the fibers and PLA matrix. Figure 10 represents the interfacial shear strength (IFSS) of PALF and CF after three different chemical treatments used in this work. As it is clear from this figure, the NaOH treatment improves the interfacial adhesion and compatibility of the lignocellulosic fiber/PLA matrix more significantly. The observed improvement stems from the removal of the impurities such as waxes, hemicellulose and lignin, breaking the fiber bundles, enlarging the fiber surface area and increasing the roughness of fiber surface. The positive and negative effects of the fiber chemical treatments on the composite characteristics are also controlled by the fiber nature. In this research, unlike PLA/PALF biocomposites, the mechanical properties of PLA/CF biocomposites do not considerably improve by various chemical treatments of lignocellulosic fibers (Siakeng et al. 2018).

5.2 Organosilane Treatment Organosilanes are the main group of the coupling agents, which are employed in industrial scales to improve the adhesion between glass fibers and polymeric

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Fig. 9 SEM images of: a Untreated CF, b CF treated by NaOH, c CF treated by silane, d CF treated by Ca(OH)2, e untreated PALF, f PALF treated by NaOH, g PALF treated by silane, h PALF treated by Ca(OH)2. Reproduced with permission from (Siakeng et al. 2018)

matrices (Zimniewska and Wladyka-Przybylak 2016). The molecules of these effective coupling agents must contain different functional agents in order to develop chemical bonding with both phases, namely polymeric matrix and reinforcing fibers (Zhang et al. 2017). Alkoxysilanes, among organosilanes, have the general structure of X–R1 –Si(OR2 )3 . Depending on which atomic groups are substituted for R1 and R2 , this group of materials is very diverse and widely used to improve the interfacial properties of the lignocellulosic fiber/polymeric matrix biocomposites (González et al. 2012; Huda et al. 2008; Orue et al. 2018; Way et al. 2011, 2012). The OR2 group

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Fig. 10 Interfacial shear strength of treated and untreated coir fibers (CF) and pineapple leaf fibers (PALF). Adapted to (Siakeng et al. 2018)

is a hydrolyzable alkoxy group that can be hydrolyzed in an aqueous environment (Kozlowski and Wladyka-Przybylak 2004). One may know that the siloxane alkoxy groups of the coupling agents can react directly with silanol groups of the glass fiber surfaces in glass fiber-reinforced composites. Unlike that, the lignocellulosic fiberreinforced composites need some traces of water in the reaction medium in order to form silanol groups in the coupling agent structure. Afterwards, the formed silanol groups participate in the condensation reaction with the hydroxyl groups of the fiber surface (Zhang et al. 2017). Moreover, the X end group in the general structure of the alkoxysilane molecule is an organic functional group that can react with the polymer matrix chain and create covalent bonds between the lignocellulosic fibers and polymer matrix. Figure 11 depicts the proposed reaction scheme between silane coupling agents, lignocellulosic fibers and PLA matrix. Through the silanization chemical treatment, first the alkoxysilane undergo hydrolysis with water and form silanol groups (step I). Then, the silanol groups in step II participate in condensation reaction with the hydroxyl groups present on the fiber surface. Finally, by choosing a proper functional group X in step III, the chemical reaction between the polymer chain and the grafted silane molecule becomes possible (Zhang et al. 2017). The sinalization treatment minimizes the destructive effects of moisture on the properties of natural fiber-reinforced composites. Moreover, this treatment improves the fiber/matrix interfacial adhesion and correspondingly the composite strength owing to the formed covalent bonding between the matrix chains and reinforcing fibers. The effectiveness of this treatment method on the composite properties depends on the employed organosilane type, treatment solution concentration, reaction temperature and time, moisture content, fiber volume fraction in the biocomposite and reaction medium (water, aqueous solvent or mixture) (Orue

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Fig. 11 Proposed schematic reaction pathway for the silanization of lignocellulosic fibers (steps I and II) and grafting onto the PLA chains (step III)

et al. 2018; Zimniewska and Wladyka-Przybylak 2016). The water content in silane treatment is a key factor. The insufficient amount of water leads to the formation of an incomplete monolayer on the fiber surface, while the high water content results in silane polymerization reaction (González et al. 2012). The alkali treatment is commonly applied before the silane modification. In this case, after the creation of silanol groups, more reactive sites on the cleaned fiber surface are accessible to participate in the condensation reactions. The fiber/matrix interfacial bonding is reportedly much more stronger in the bicomposites that undergo silanization surface modification after alkali pretreatment, thereby enhancing the composite tensile strength (Zhang et al. 2017). A literature review on the use of this treatment in PLA-based biocomposites reveals that the silane treatment conditions have not been controlled well. Therefore, the biocomposite properties have changed in wide ranges, repeatedly resulting in a limited improvement of interfacial strength and other properties (González et al. 2012; Le Moigne et al. 2014; Siakeng et al. 2018). Nevertheless, this treatment has been frequently used to enhance the interfacial adhesion and compatibility of different lignocellulosic fibers in the PLA matrix (Huda et al. 2008; Jandas et al. 2012; Ander Orue et al. 2016; Song et al. 2018; Way et al. 2011, 2012). In one of these studies (González et al. 2012), the PLA biocomposites were prepared with spent kraft and sisal fibers as reinforcement agents through twin-screw extruder melt-compounding and injection molding shaping processes. Different types of silane coupling agents were applied in this work to improve the

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thermal and mechanical properties of the PLA-based biocomposites, including 3aminopropyltriethoxysilane and 3-(trimethoxysilyl)propyl methacrylate. No significant improvement was observed for any of the studied mechanical properties of the obtained biocomposites after silane treatment (González et al. 2012). Similarly, another research group (Chun et al. 2012) evaluated the effects of coconut shell powder (CSP) on the thermal, mechanical and morphological properties of the PLA biocomposites before and after the fiber silane treatment. A silane coupling agent, 3-aminopropyltriethoxysilane, at concentration of 3% (v/v) in ethanol was applied for treating the filler surface. It is obviously evident in Fig. 12 that the untreated CSP particles are not properly embedded in the PLA matrix and some micro-voids (marked by white arrows) are also discernable at the fractured surface of the bicomposite. At high fiber loading (60 phr), some agglomerates of the untreated CSP particles are also distinguishable. On the other hand, after treating the filler surface with silane coupling agent, the dispersion state of the solid particles becomes more homogenous. As a direct result, the elastic modulus, tensile strength, T g and crystallinity degree of matrix improve comparing to the untreated CSP-filled biocomposites (Chun et al. 2012).

Fig. 12 SEM image of tensile fractured surface of: a PLA/untreated coconut shell powder (CSP) biocomposite at a filler loading of 30 phr, b PLA/untreated CSP biocomposite at a filler loading of 60 phr, c PLA/silane-treated CSP biocomposite at a filler loading of 30 phr and d PLA/silane-treated CSP biocomposite at a filler loading of 60 phr. Reproduced with permission from (Chun et al. 2012)

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Fig. 13 a Chemical structure of the used organosilane coupling agent and b proposed silane-grafted fiber/PLA structure. Adapted to (Le Moigne et al. 2014)

More improvements in the mechanical properties of the lignocellulosic filled PLA systems are achievable by optimization of the silane reaction conditions. In this regard, the organosilane treatment method was applied to improve the properties of the PLA/short flax fiber biocomposites (Le Moigne et al. 2014). The effects of different influential parameters of the treatment process including the organosilane solution composition, reaction temperature, fiber soaking time, heating temperature and alkali pretreatment were investigated in this study. To treat the flax fibers, a silane coupling agent, (3-glycidyloxypropyl)trimethoxysilane, with chemical structure shown in Fig. 13 was employed. By alkali pretreatment, the amount of hydroxyl groups on fiber surfaces as the potential reactive sites for the silane grafting reaction is heightened. Furthermore, the fiber surface roughness also increases that has a positive impact on the fiber/matrix mechanical interlocking. By these mechanisms, the alkali pretreatment provokes improvements in the Young modulus, yield stress and impact strength of biocomposite, particularly comparing with the PLA/silane-treated flax biocomposite (Fig. 14). The silanization treatment of flax fibers, regardless of the alkali pretreatment, leads to a reduction in the fiber hydrophilicity and an increment in the fiber/matrix interfacial adhesion. As can be found in Fig. 14, the organosilane treatment process raises the Young modulus, yield stress, impact stress and equivalent interphase thickness of the PLA-based biocomposite containing untreated fibers. Further studies on the failure mechanisms at the fiber/matrix interface disclose the fact that the adhesive interfacial failure occurs for the untreated fiber-reinforced biocomposite, in which the crack propagation path goes through the matrix as well as fiber/matrix interface. In marked contrast, for the silane-treated fiber/PLA biocomposites, cohesive interfacial failure is observed, which is accompanied by fiber breakage and matrix tearing. Besides, throughout the stress exposure and crack propagation, the silane-treated fibers remained bonded to the PLA matrix. In fact, the observed improvement in the fiber/matrix adhesion over the silanization treatment is induced by the following mechanisms: (I) Chemical coupling, which is formed by

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Fig. 14 a Young modulus and yield stress, b impact strength and equivalent interphase thickness of: neat PLA, PLA/untreated flax fiber biocomposite, PLA/treated flax fiber biocomposite with organosilane modification at a soaking bath temperature of 70 °C (Silane), PLA/treated flax fiber biocomposite with alkali pretreatment and organosilane modification at a soaking bath temperature of 20 °C (NaOH+silane). Adapted to (Le Moigne et al. 2014)

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the esterification reaction between the epoxy functional groups of the grafted silane molecules and the end groups of PLA chains. (II) Physical interactions including Van der Waals forces and direct hydrogen bonding between the functional groups of the fibers and PLA chains. The covalent bonds formed between the silane coupling agent, flax fiber surface and PLA matrix chains are depicted in the chemical structure shown in Fig. 13 (Le Moigne et al. 2014). The beneficial effects of alkali pretreatment on the silane chemical treatment is clearly shown in a study (Zafar et al. 2016). In this research, the PLA/jute fiber biocomposites were prepared using untreated fibers and treated fibers with alkali and silanization treatment. As mentioned, the alkali treatment helps the lignocellulosic fibers to participate in the grafting reaction with the silane coupling agents more extensively. The related mechanism involves in increasing the fiber surface cleanliness and providing more accessible cellulosic hydroxyl groups at the fiber surface. Therefore, one may conclude that the wettability and functionality of the fiber surface are intensified by the pretreatment. The success of the treatment reactions were examined by Fourier-transform infrared (FTIR) spectroscopy and SEM observations. The disappearance of the absorption peak at 1739 cm−1 (attributed to the C=O bond of carboxylic and ester groups of hemicellulos), and broadening the absorption peak at 3427 cm−1 (related to –OH groups) in the FTIR spectrum of untreated fibers after the chemical treatment provide conclusive evidence on the removal of hemicellulose, pectin, fat and oils through the alkali modification and correspondingly, an increment in the fiber surface functionality. In the FTIR spectrum of the silane-treated fibers, the presence of a peak at 776 cm−1 (related to the symmetric stretching vibration of –Si–C- bond), and 1200 cm−1 (related to the –Si– O–C- bond) corroborates the occurrence of the condensation reactions between the fiber hydroxyl groups and silanol groups. It has been reported that the fiber crystallinity degree rises to higher levels after alkali treatment owing to the removal of the non-cellulosic impurities. On the other hand, the fiber crystallinity degree diminishes after silane treatment due to the augmentation in the amorphous cellulose content. The results of tensile test on a single fiber show noticeable increases in the tensile modulus and strength of jute fiber after alkali treatment, whereas the subsequent exposure of alkali-treated fibers to the silane solution in silanization modification worsens the fiber properties. The given explanation is the removal of the soft materials present on the fiber surface after alkali treatment, while the silane treatment in an acidic medium eventuates in the cleavage of β-1,4-glycosidic linkages, hence reducing the fiber strength. In spite of that, the best PLA/jute fiber interfacial strength is reported for the biocomposite containing the jute fibers treated with coupled alkali and silane modifications. The strong fiber/matrix interface for the silane/NaOH treated fibers substantially promotes the interfacial shear strength of the embedded fiber in PLA matrix compared to the untreated fiber (an increment about 112%) (See Fig. 15). In fact, the synergetic effects of the alkali and silanization processes provoke the observed enhancement and considerably shorter pull-out fiber from the PLA matrix (Fig. 15) (Zafar et al. 2016).

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Fig. 15 Interfacial shear strength and critical fiber length for: PLA/untreated jute fiber composite, PLA/treated jute fiber composite using alkali modification and PLA/treated jute fiber composite using alkali pretreatment and silane modification. Adapted to (Zafar et al. 2016)

5.3 Esterification Treatment Because of the extensive access to the hydroxyl groups available on the surface of lignocellulosic fibers, the esterification treatment has been employed as a suitable treatment method to improve the interfacial interactions in natural fibers/PLA biocomposites. Unlike silane treatment, in which the interfacial coupling is created through the covalent bonding formation between lignocellulosic fibers and matrix, the esterification treatment is done only by functionalizing the fiber surfaces. Over the esterification treatment, the hydrophilic hydroxyl groups are transformed into atomic groups with more hydrophobicity through the ester bonding. Thus, the hydrophobicity of fiber surfaces is intensified and the compatibility between the plant fibers and polymer matrix improves (Zhang et al. 2017). The esterification reaction normally involves the reactions of organic acids or anhydrides with the fiber hydroxyl groups. The proposed reactions of the hydroxyl groups of the lignocellulosic fibers and anhydride/carboxylic acid groups are depicted in Fig. 16. Different types of esters, depending on the nature of the employed organic acids in anhydride molecules, can be applied in the esterification modification process. The esters with one to four carbon atoms are formate, acetate, propionate and butyrate. Other esters including laurate and stearate contain 12 and 18 carbon atoms, respectively. Furthermore, dicarboxylic esters with double bond in the carbon chain structure like fumarate and maleate are also employed in the esterification treatment (Kozlowski and Wladyka-Przybylak 2004; Zimniewska and Wladyka-Przybylak 2016). It is noteworthy that the longer alkyl group attached to the

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Fig. 16 Proposed reaction scheme of the hydroxyl groups of lignocellulosic fiber with a carboxylic acid and b anhydrides. Adapted to (Abu Ghalia and Dahman 2017; Way et al. 2011)

ester group gives the lignocellulosic fiber more thermoplasticity. The encountered disadvantage of this treatment method is the requirement of using dried fibers in order to increase the reaction efficiency (Kozlowski and Wladyka-Przybylak 2004). Depending on the applied chemical type to create ester bonding, the acetylation, benzoylation or maleation treatment methods are used to intensify the hydrophobicity of the fiber surface and decrease the fiber moisture uptake (Zhang et al. 2017).

5.3.1

Acetylation Reaction

The acetylation process using acetic anhydride is reported as the most common method to modify wood (Abu Ghalia and Dahman 2017). In recent decades, this method has been also applied on an industrial scale for the treatment of lignocellulosic materials. Various chemical compounds, such as acyl chloride, acetic acid and acetic anhydride, are employed in the acetylation process of lignocellulose (Zhang et al. 2017). One may know that the hydrolysis reaction of acetic anhydride to acetic acid is relatively a highly exothermic rapid reaction and the reaction of fibers with acetic anhydride produces acetic acid as a by-product, which must be removed after the fiber surface treatment (Shafiei et al. 2015). In the acetylation process, using acetic anhydride, the R2 group (shown in Fig. 16b) is substituted by –CH3 group. Researches have verified that the substitution reaction of acetyl groups with the hydroxyl groups of the amorphous components of lignocellulosic fibers like lignin and hemicellulose is more likely than the reaction with the hydroxyl groups of highly crystalline cellulose. The diffusion constant of the compounds containing acetyl groups is much lower in the closely packed area of the crystals than the amorphous parts. To confront the problem, the plant fibers are commonly treated with alkali solution first (Shafiei et al. 2015). The substitution reaction of fiber contributes to the fiber weight gain owing to

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the substitution of the fiber hydroxyl groups with acetyl groups. Since this treatment involves the replacement of the hydrophilic hydroxyl groups by the ester groups with more hydrophobicity, the treated fibers are more hydrophobic and the equilibrium moisture content of fibers diminishes (Kozlowski and Wladyka-Przybylak 2004). One may use catalysts to increase the acetylation reaction rate. The most frequently used catalysts for the acetylation reactions are pyridine, sulfuric acid and potassium and sodium acetate. In case of using a strong acid as catalyst, the proper choice of reaction conditions for the effective fiber treatment is critical, since the reaction can cause the degradation of fiber microstructure and consequently, the deterioration of fiber properties (Orue et al. 2018). The acetylation treatment of the lignocellulosic fibers is also known as a process for fiber plasticization (Siakeng et al. 2019a). In a research (De Rosa et al. 2011), the effects of acetylation treatment was investigated, using acetic acid, on the mechanical properties of the okra fibers. It was found that the tensile strength and Young modulus of fibers drop to lower values after treatment. The fiber surfaces become cleaner relative to the primary fibers and the fiber hydrophilicity decreases after acetylation treatment, resulting in better wettability between the fibers and hydrophobic resins (Orue et al. 2018). Figure 17 illustrates the SEM images of the cryofractured surfaces of the PLA/rice straw fiber (RSF) biocomposites in the presence of poly (ethylene glycol) (PEG) as a plasticizer. The left- and right-hand columns relate to the biocomposites containing untreated and acetylated fibers, respectively. For preparation the PLA/RSF biocomposites, the acetylation treatment of fibers was performed using acetic acid and acetic anhydride, accompanied by sulfuric acid as a catalyst. The images of the biocomposites containing untreated fibers demonstrate that fiber pull-out and fiber/matrix debonding occur extensively, whereas these phenomena are observed to a lesser extent on the fractured surface the biocomposite containing acetylated RSF. Additionally, the surfaces of treated fibers are covered with a layer of PLA matrix indicating improved interfacial adhesion and fiber hydrophobicity. The plant fiber-reinforced PLA biocomposites generally show lower moisture gain by replacing the hydroxyl groups with more hydrophobic acetyl groups. In order to diminish the biocomposite sensitivity to moisture, maple fibers (MF) were treated by silanation and acetylation processes after alkali pretreatment (Way et al. 2011). Their findings show that among different treated fibers including untreated MF, treated MF with NaOH, treated MF with silane/NaOH, and treated MF with acetylation/NaOH, the lowest natural moisture content relates to the last one, the fibers treated with acetylation and alkali methods. This property of treated fibers leads to less moisture absorption of the treated fiber-reinforced PLA composites. The meltcompounding in a twin-screw extruder and shaping in an injection molding machine were applied to prepare the PLA/MF biocomposites. Throughout the melt processes, the PLA chains were hydrolyzed and undergone chain scission despite being dried. By the melt processing, the weight-average molecular weight (M w ) of PLA chains in the processed pure PLA and PLA/untreated MF biocomposite diminished by 31% and 41%, respectively (Fig. 18). The residual moisture in the samples assists the hydrolysis reaction of the PLA chains. As a result of less water absorption, the lowest

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Fig. 17 SEM images of the cryofractured surfaces of PLA/untreated RSF biocomposites (micrographs on the left-hand side) and PLA/acetylated RSF biocomposites (micrographs on the right-hand side) containing 6 phr of PEG at a filler loading of: a and b 10 phr, c and d 20 phr, e and f 30 phr. Author’s unpublished data

loss in the PLA molecular weight is determined for the biocomposite containing the acetylated and alkali-treated fibers (Way et al. 2011). Furthermore, they reported that different properties for tensile and flexural strength were obtained for the PLA biocomposites containing alkali-treated fibers depending on the NaOH solution concentration and treatment reaction time. In this work, it is shown that the tensile and flexural strength of the PLA/MF biocomposite respectively

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Fig. 18 Weight- and number-average molecular weight (Mw and Mn) of unprocessed raw PLA as well as processed PLA and PLA/MF biocomposites. Adapted to (Way et al. 2011)

decrease by 17 and 16% by using the alkali treatment of fibers. The observed reductions are attributed to the easier initiation of crazes at the fiber/matrix interfaces and fiber tenacity loss over the alkali modification process. Nonetheless, the acetylation process after alkali pretreatment can compensate for the lost strength to some extent, owing to better wettability of the acetylated fibers with PLA matrix and improved fiber/matrix interfacial adhesion. In spite of the improvements, the consecutive treatment processes, i.e. alkali and acetylation reactions, do not result in an improvement in the flexural and tensile strength of the PLA/untreated MF composites (Way et al. 2011). In a similar study (Jonoobi et al. 2012), the acetylation treatment was employed on the surface of cellulose nanofibers (CNF), which were obtained from the raw kenaf bast fibers. In this research, the fiber pulp was first separated from the kenaf bast fibers by NaOH and bleaching treatments. Afterwards, the acetylation process was performed on the pulp by using acetic anhydride accompanied by the pyridine catalyst. Then, the prepared fibers were dispersed in water and exposed to shear mixing to obtain a suspension of nanofibers. On the FTIR spectra of untreated CNF and acetylated cellulose nanofibers (ACNF), a broad absorption peak at 3330 cm−1 and another peak at 1640 cm−1 are attributed to the hydroxyl groups and absorbed water of cellulose nanofibers, respectively (Fig. 19). The intensity of these peaks is substantially decreased by the acetylation treatment. In addition, the appearance of two peaks in the FTIR spectrum of ACNF at 1744 cm−1 and 1238 cm−1 , which are respectively related to the stretching vibration of C=O bond of carbonyl in ester groups and C–O bond of acetyl groups, verifies the successful substitution reaction of the hydroxyl groups with the acetyl ones.

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Fig. 19 FTIR spectra of untreated and acetylated cellulose nanofibers (CNF and ACNF). Adapted to (Jonoobi et al. 2012)

It is worthy to note that, after the acetylation process, the water contact angle of fibers increases from 33° to 115°, which reveals the improved hydrophobicity of the treated cellulose fibers. Moreover, the addition of cellulose nanofibers to the PLA matrix increases the storage modulus and decreases tan δ, albeit the variations of storage modulus and damping factor are higher for the acetylated nanofibers. The larger changes show that the acetylated fibers have stronger interactions with the PLA chains and restrict the PLA molecular motions more extensively. Despite that, the crystallinity and thermal stability of the cellulose nanofibers diminish after the acetylation modification. Therefore, the acetylation treatment does not bring about an improvement in the Young modulus and tensile strength of the PLA/untreated CNF biocomposites, though the interfacial adhesion between ACNF and PLA matrix improves (Jonoobi et al. 2012).

5.3.2

Maleation Reaction

Maleate coupling agents are capable of creating effectual chemical bonds between plant fibers and matrix chains. Maleation treatment can be carried out following two procedures. One method is to pretreat lignocellulosic fibers with a maleated polymer. Then, the treated fibers is compounded with polymer matrix. In the second method, plant fiber, polymeric matrix, maleic anhydride (MA) and a peroxide initiator are compounded together via reactive extrusion process, which is a single step one. Afterwards, the prepared compounds are molded into final products by compression or injection molding processes (Zimniewska and Wladyka-Przybylak 2016).

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Regardless of the procedure used, to improve the interfacial adhesion of PLA and lignocellulosic fibers, MA grafting reaction must be performed on PLA chains. According to Fig. 20, first the peroxide initiator, which is normally used in maleation treatment methods, creates reactive sites on PLA chains through dissociation and radical transfer reactions. Second, MA molecules are grafted onto PLA chains and form maleic anhydride-grafted PLA (MAPLA) macromolecules. Subsequently, MA functional groups in MAPLA molecules react with the hydroxyl groups of plant fibers by the occurrence of MA ring-opening reaction. The reaction mechanism between MAPLA chains and lignocellulosic fibers involves the thermal activation of graft copolymer and subsequent esterification reaction with lignocellulosic fibers. Thus, the chemical reaction between fibers and PLA chains becomes possible (Orue et al. 2018). Hence, depending on the correct choice of processing conditions, the attainment of higher fiber/matrix compatibility is then within reach (Zhang et al. 2017). The surface energy of lignocellulosic fibers decreases and is put in the range of PLA surface energy by the treatment. As a result, higher wettability of natural fibers is provided by hydrophobic matrix and the interfacial strength improves (Siakeng et al. 2019a; Orue et al. 2018). Maleation treatment has been used for the chemical modification of the interfaces of PLA-based biocomposites containing different natural fibers such as triticale straw, wheat straw, bamboo and banana fibers (Mihai and Ton-That 2014; Nyambo et al. 2011; Posada et al. 2016; Majhi et al. 2010). Nyambo et al. (2011) in some related studies, MAPLA chains were first obtained via melt mixing process using PLA,

Fig. 20 a–c Schematic chemical drawing of maleic anhydride grafting reaction onto PLA chains and d grafting reaction of MAPLA on plant fibers. Adapted to (Zhang et al. 2017; Rigolin et al. 2019)

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MA, and initiators like 2,5-bis(tert-butylperoxy)-2,5-dimethylhexane (Nyambo et al. 2011; Mihai and Ton-That 2014). Then, in the second mixing process, the resultant compatibilizer was applied in compounding of PLA/natural fiber biocomposites. Their findings reveal that the tensile and flexural modulus and tensile and flexural strength of PLA/untreated fiber biocomposites are improved by adding MAPLA compatibilizer (Mihai and Ton-That 2014; Nyambo et al. 2011). As mentioned earlier, it is possible to achieve the maleation treatment, both the grafting reaction of MA molecules onto PLA chains assisted by an initiator and ringopening reaction of MA with fiber hydroxyl groups, through a single step melt-mixing process. It was reported that the tensile modulus and strength of PLA/bamboo fiber (BF) biocomposites were improved by performing this reactive extrusion process (Posada et al. 2016). However, it has been found that the treatment agent of maleic anhydride in the reactive mixing of PLA/natural fiber biocomposites has an optimum value, above it, the composite properties are deteriorated (Majhi et al. 2010). In this study, the interfacial properties of PLA/alkali-treated banana fibers biocomposites were improved using reactive mixing of components in an internal mixer at MA levels of 1, 3 and 5 wt%. It was determined that the highest improvements in the biocomposite properties such as tensile modulus and strength as well as impact strength were achieved at 1 wt% of MA content (Fig. 21) and further increments in MA level led to final properties deterioration. In fact, adding MA improves the properties by creating chemical linkages between PLA chains and fiber surfaces and proper matrix/fiber bonding. By adding more MA to the melt process, the MA content would reach the critical interfacial concentration of the compatibilizer and saturate the matrix/fiber interfaces. Beside the mentioned properties, the maximum

Fig. 21 Maleic anhydride effect on some mechanical properties of PLA/untreated bamboo fibers (UFB) biocomposites. Adapted to (Majhi et al. 2010)

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values of T g , T c , HDT and storage modulus of treated biocomposites by maleation process were obtained at 1 wt% of MA content. In similar studies, MAPLA grafted chains were first obtained in twin-screw extruders, using dicumyl peroxide initiator (Kim et al. 2012; González-López et al. 2019). Afterward, 3 wt% of MAPLA were added to PLA/agave fiber, PLA/cassava flour and PLA/pineapple flour biocomposites during extrusion process. The incorporation of MAPLA compatibilizer into melt, during extrusion process, results in the esterification reactions of grafted MA, on the PLA backbone, with hydroxyl groups of lignocellulosic materials (Fig. 20d). In fact, covalent linkages and hydrogen bonding between matrix and the reinforcing filler along with the fiber hydrophobicity reduction over maleation process bring about strong matrix/fiber interfaces. This, in turn, led to relative increment in modulus, strength and hardness of the maleated PLA/agave fiber biocomposites and also relative decrease of impact strength, water absorption and water diffusion coefficient of these biocomposites (Fig. 22) (González-López et al. 2019). In another study, the augmentations in storage modulus, HDT and PLA crystallinity degree through MA esterification reaction in PLA/fiber composite were observed (Kim et al. 2012). The SEM image of the fractured surface of PLA/agave fiber biocomposites, containing untreated fibers, shows intense fiber pull-out and large quantity of debonded fibers (Fig. 23). At higher magnifications, some interfacial gaps are evident,

Fig. 22 Mechanical, thermo-mechanical and physical properties of untreated agave fiber/PLA biocomposites (UFBC) and treated agave fiber/PLA biocomposites (TFBC) against fiber loading. Adapted to (González-López et al. 2019)

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Fig. 23 SEM micrographs of: a and c untreated agave fiber/PLA biocomposites (UFBC), b and d treated agave fiber/PLA biocomposites (TFBC) at a fixed fiber loading of 20 wt%. Reproduced with permission from (González-López et al. 2019)

indicating matrix/fiber debonding. On the other hand, the fiber/matrix compatibility of maleated PLA/agave fiber composites was improved and effective stress transfer across matrix/fiber interfaces was obtained. The SEM images of these biocomposites clarify that the failure mostly occurs at the bulk of fibers and matrix, not the interfaces. Therefore, the observed change in the failure mechanism led to a reduction in the impact strength of PLA/treated agave fiber biocomposites compared to PLA/untreated fiber biocomposites, since fiber pull-out requires more dissipated energy during failure (González-López et al. 2019). Further review of the obtained results in Figs. 21 and 22 and other similar works (Nyambo et al. 2011; Kim et al. 2012), reveals that the esterification treatment with MA has led to some improvements in the properties of PLA/untreated fiber biocomposites, though the properties of MA-treated PLA/fiber biocomposites have worsened

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Fig. 24 Main reactions of MA with PLA chains in the presence of organic peroxide that induce an increment in the MAPLA acidity. Adapted to (Rigolin et al. 2019)

comparing with the analogous properties of virgin PLA. It is noteworthy that, despite the improvements in fiber/matrix interfaces by meleation reaction, the acidity of PLA chains increases by MA use. It is in contrast to the MA grafting reaction of polyolefins through reactive extrusion process. The acidity increment of PLA chains over MA grafting reaction was reported up to 342% in one study (Rigolin et al. 2019). According to Fig. 24, the acidity increase of PLA chains by MA compatibilizer is probable through three reaction pathways: (a) insertion of MA molecules onto PLA chains, (b) condensation reaction of MA anhydride groups with hydroxyl end groups of PLA, and (c) chain scission of PLA macromolecules during reactive extrusion process in the presence of organic peroxide. The occurrence of side reactions along with MA grafting reaction onto PLA backbone leads to the acidity rise of MAPLA. Furthermore, during the melt-mixing process of PLA-based biocomposites containing lignocellulosic fibers, the degradation of PLA chains is likely through hydrolysis reaction, owing to the fiber residual moisture. The hydrolysis reaction of PLA chains is catalyzed by acidic sites. Consequently, PLA molecular weight reduces more considerably in the presence of MAPLA chains. Figure 25 illustrates the reduction of PLA molecular weight in the presence of coir fibers (CF) by adding 5 and 10 wt% of MAPLA. More significant decrease of PLA molecular weight, which stems from the presence of MAPLA compatibilizer, deteriorates the reinforcement effect of maleated fibers, despite stronger PLA/fiber interfaces and interfacial covalent bonding (Rigolin et al. 2019). These findings confirm the necessity of selecting the appropriate processing conditions of MA grafting reaction onto PLA chains to improve the properties of PLA/fiber biocomposites more effectively.

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Fig. 25 Changes in average molecular weights of PLA at different coir fiber and MAPLA contents over a melt-mixing process. Adapted to (Rigolin et al. 2019)

5.3.3

Benzoylation Reaction

Benzyl chloride is frequently applied as a chemical agent for reducing the hydrophilicity of lignocellulosic fibers and improving the fiber/matrix compatibility. In benzoylation treatment process, plant fibers are exposed to an aqueous NaOH solution before the fiber reaction with benzoyl chloride to provide more accessible hydroxyl groups of fibers for substitution reaction. After alkali pretreatment, the natural fibers are then modified by benzoyl chloride through esterification reaction (Orue et al. 2018). The reaction scheme of benzoylation treatment is shown in Fig. 26. By mercerization pretreatment and benzoylation reaction, the impurities are removed from the lignocellulosic fiber surfaces and fiber roughness increases. Furthermore, by substitution of benzoyl groups for proton of hydroxyl groups, the fibers become more hydrophobic. In addition, fiber/matrix interfacial adhesion improves and correspondingly, the thermal stability and strength of plant fiber-reinforced composites are enhanced (Siakeng et al. 2019a).

Fig. 26 Esterification reaction of the fiber hydroxyl groups with benzoyl chloride

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The aforementioned improvements have been reported for woven jute fabricreinforced PLLA biocomposites, which were prepared through hot-press molding process, in both weft and warp directions (Khan et al. 2016). In this research, the benzoylation treatment of fabrics resulted in noticeable enhancements in the mechanical characteristics including tensile modulus and strength, flexural modulus and strength, impact strength and elongation at break of virgin PLLA and PLLA/untreated fabrics biocomposites in both warp and weft directions. Figure 27 illustratively shows the mentioned improvements of mechanical properties for PLLA/jute fabrics composites. For other geometric shapes of the reinforcement agents based on plants, the beneficial effects of benzoylation treatment on properties of PLA/olive husk flour (OHF)

Fig. 27 Mechanical characteristics of virgin PLLA and PLLA/woven jute fabric biocomposites after and before benzoylation treatment. Adapted to (Khan et al. 2016)

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biocomposites were investigated and the results were compared with more common chemical modifications including alkali and silane treatments (Isadounene et al. 2017). To treat OHF through benzoylation reaction, the reinforcement agent was first treated by NaOH, then filler particles were exposed to benzoyl chloride solution. Among different chemical modifications, i.e. alkali, silane and benzoylation reactions, the incorporation of treated OHF particles with benzoyl chloride into PLA matrix did not lead to the best mechanical performance. As can be observed in Fig. 28, the largest Young modulus, tensile strength and elongation at break are comparatively obtained for the PLA biocomposites containing alkali-treated OHF filler. In contrast, for most cases, PLA/OHF biocomposites having benzoylated reinforcement agent show weaker mechanical characteristics comparing with other PLA/treated filler biocomposites at the same OHF loading. The researchers reported that the mercerization treatment had more profound impact on the improvement of matrix/filler interfacial adhesion than silane and benzoylation modifications, through the removal of impurities and increment of filler surface roughness. Moreover, the highest storage modulus and thermal stability were also determined for the biocomposites containing alkali treated OHF particles. Throughout benzoylation process, the hydroxyl groups of plant fibers are transformed into the benzoate groups. Therefore, the quantities of inter- and intramolecular hydrogen bonds among fiber components diminish to lower values and correspondingly, the natural fibers become more hydrophobic. In a similar manner to acetylation treatment, benzoylation process gives the lignocellulosic fibers thermoplasticity. Reportedly, the reinforcing effect of benzoylated plant fibers in PLA matrix is controlled by the concentration of benzoyl chloride solution after alkali pretreatment (Al-Mobarak et al. 2018, 2019). In the related works, sponge-ground fibers (SGF) obtained from luffa cylindrica plant were applied as lignocellulosic reinforcement agent. The influences of three different chemical treatments, i.e. alkali, acetylation and benzoylation processes, on the mechanical, physical, thermal and biodegradation properties as well as antibacterial activity of PLA-based biocomposites were studied. As can be found in Fig. 29, the highest compression strength was achieved for PLA/benzoylated SGF biocomposite among others, when the treatment aqueous solutions, namely benzoyl chloride, acetic anhydride and NaOH solutions, at a fixed concentration of 5 wt% were applied. Nevertheless, the compression strength of different PLA/treated SGF biocomposites were measured to be close to each other, when the solutions of treatment agents were used at a constant concentration of 10 wt%. The dependence of the properties of PLA-based green composites on the concentration of treatment agent solutions can be interpreted by studying the morphology and interfacial phenomena of the biocomposites after chemical reactions. The SEM images of fractured surfaces in Figs. 30 and 31 reveal the existence of poor matrix/fiber interface and lots of pores on the interfacial surface of PLA/untreated SGF composite. By fiber modifications using the solutions at a treatment agent concentration of 5 wt%, the matrix/plant fiber interfacial adhesion improves and the fractured surfaces show lower roughness and smaller cracks. As can be observed

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Fig. 28 Mechanical characteristics of virgin PLA and PLA/olive husk flour biocomposites after and before different chemical treatments including alkali, silane and benzoylation modifications. Adapted to (Isadounene et al. 2017)

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Fig. 29 Effects of the concentration of the treatment agent solutions on the compression strength of PLA/SGF biocomposites at a fixed filler content of 5 wt%. Adapted to (Al-Mobarak et al., 2018; 2019)

in Fig. 30, the lowest surface roughness belongs to the fractured surface of PLA/benzoylated fiber composite. However, an increment in the concentration of treatment agent to 10 wt% leads to the fractured surfaces with more similarity for all biocomposites, which can be seen in Fig. 31. At this concentration, the matrix/fiber interfacial adhesion is enhanced more, hence eliminating the voids. The concentration of treatment agents resulted in similar trends in the biodegradation rate in soil, T g and T m of the prepared green composites. The highest thermal stability and biodegradation rate along with the largest T g and T m were attained for PLA/benzoylated SGF composite, in which lignocellulosic fibers were exposed to the benzoyl chloride solution at 10 wt% concentration. Additionally, the antibacterial activity tests using a category of Vero cell line reveal that the benzoylated fiberreinforced composites do not show any cytotoxicity. At the biocomposite surface, non-cancer cells grow indicating the harmlessness of the prepared biocomposite for human body in biomedical purposes (Al-Mobarak et al., 2018, 2019).

5.4 Bleaching Treatment Bleaching is a common method for discoloration and whitening of lignocellulosic fibers. Normally, bleaching agents such as hydrogen peroxide (H2 O2 ), sodium chlorite (NaClO2 ) and sodium hypochlorite (NaClO) are used in this method (Zhang et al. 2017). The bleaching agents react with the hydroxyl groups of lignin and

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Fig. 30 SEM micrograph of the fractured surface of PLA/SGF composite containing: a untreated fibers, b alkali-treated fibers, c acetylated fibers and d benzoylated fibers using the treatment agent solutions at a constant concentration of 5 wt%. (Al-Mobarak et al. 2018) Open Access

degrade the impurity of plant fibers. For instance, NaClO2 releases chlorous acid after acidification reaction. After oxidation, the chemical forms chlorine dioxide (ClO2 ). Afterwards, ClO2 reacts with lignin and removes it from lignocellulosic fibers (Siakeng et al. 2019a). Hydrogen peroxide also, as a bleaching agent, undergoes dissociation reaction in alkaline media and forms perhydroxyl ions (HOO- ) (Fig. 32). The ions attack light absorbing chromophoric groups of lignin and hemicellulose including carbonyl and conjugated carbonyl groups as well as quinones and remove them from fibers after treatment (Marwah et al. 2014). Different studies have employed bleaching method to enhance the properties of PLA/plant fiber biocomposites (Posada et al. 2016; Hou et al. 2006; Razak et al. 2014). Bleaching process is accompanied with delignification and increases the fiber brightness. On the other hand, the delignification and dewaxying reactions heighten the fiber roughness through bleaching process, which can strengthen the matrix/fiber interfaces by providing mechanical interlocking (Zhang et al. 2017). Bleaching process not only can improve the mechanical performance of natural fiber-reinforced biocomposites, but also it can lead to an improvement in the physical appearance of biocomposites. The incorporation of lignocellulosic fibers into polymer matrices generally eventuates in dark brown products that limits the product applications. As

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Fig. 31 SEM micrograph of the fractured surface of PLA/SGF composite containing: a untreated fibers, b alkali-treated fibers, c acetylated fibers and d benzoylated fibers using the treatment agent solutions at a constant concentration of 10 wt%. (Al-Mobarak et al. 2018) Open Access

Fig. 32 Reaction scheme of hydrogen peroxide dissociation

can be seen in Fig. 33, bleaching process overcomes the common disadvantage of lignocellulosic fiber-reinforced biocomposites, which is related to fiber brightness. This treatment method can provide a brighter PLA/plant fiber biocomposite that can be also colored by applying different color masterbatches (Marwah et al. 2014; Razak et al. 2014). As mentioned earlier, it is possible to apply hydrogen peroxide in alkaline media to bleach lignocellulosic fibers. This bleaching agent was used for treating the oil palm empty fruit bunch (OPEFB) and kenaf fibers (Marwah et al. 2014; Razak et al. 2014). In these works, the plant fibers were first treated with warm water. Then, the fibers were dipped in 50 vol% H2 O2 solution at a pH of 11 and temperature of 70 °C. As it is evident in Fig. 34, the roughness of OPEFB fibers has increased after bleaching treatment and some pores are observable on the fiber surfaces. As mentioned before, the removal of fiber impurities such as lignin, hemicellulose and waxes can cause an improvement in the fiber/matrix interfacial strength by

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Fig. 33 Upper row: untreated and bleached oil palm empty fruit bunch fibers, lower row: a virgin PLA, b PLA/untreated fiber biocomposite, c PLA/bleached fiber biocomposite, d PLA/color masterbatch/bleached fiber biocomposite. (Marwah et al. 2014) Open access

providing mechanical interlocking (Marwah et al. 2014). In this work, X-ray diffraction (XRD) studies reveal that the fiber crystallinity index has changed from 37% to 61% by bleaching modification process. Higher crystallinity degree reinforces the fibers more. Considering that and improved interfacial strength, more efficient stress transfer across fiber/matrix interfaces was attained for PLA/bleached lignocellulosic fiber biocomposites. The fractured surfaces of PLA/OPEFB biocomposites without any treatment in Fig. 35 show significant extents of fiber pull-out and interfacial gaps due to matrix/fiber debonding under load. While smaller gaps and stronger interfacial adhesion can be observable on the fractured surfaces after bleaching treatment. Besides, the tensile modulus and strength, flexural modulus and strength, impact strength and elongation at break were found to be all improved after bleaching treatment. However, one may note that different types of strength and elongation at break of the biocomposites containing bleached fibers were lower than the corresponding values of pure PLA in these researches (Marwah et al. 2014; Razak et al. 2014). Sodium chlorite was also used for the treatment of pecan nutshell (NS) lignocellulosic material in PLA matrix (Agustin-Salazar et al. 2018). In this research, NS was

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Fig. 34 SEM image of: a untreated OPEFB fibers, b and c bleached OPEFB fibers, the right column is higher magnification images of left ones. (Marwah et al. 2014) Open access

treated in aqueous solutions of acetic acid and sodium chlorite at 90 °C and hollocellulose (HC) part of the lignocellulosic material, i.e. polysaccharide constituents including cellulose and hemicellulose, was extracted (Agustin-Salazar et al. 2018). FTIR and 13 CNMR spectroscopic results confirmed the hollocellulose extraction by bleaching treatment. In 13 CNMR spectrum of HC sample, there were no peaks of aromatic lignin carbons in chemical shifts of 56, 120 and 170 ppm. Additionally, the presence of NMR peaks in the range of 60–105 ppm attributed to the carbons of cellulose and hemicellulose rings, a peak at 172 ppm related to the carbonyl carbon of acetic ester and a sharp peak at 21 ppm correlated with the methyl carbon of acetyl moiety proved that HC just had cellulose and hemicellulose components.

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Fig. 35 SEM image of fractured surface of: a virgin PLA, b PLA/untreated fibers, c PLA/bleached fibers and d PLA/bleached fibers/color masterbatch samples. The biocomposites contained a fixed fiber loading of 30 wt%. (Marwah et al. 2014) Open access

It is noteworthy that the thermal stability of the biocomposites containing HC decreased, compared to the PLA-based biocomposites containing untreated NS, despite the removal of lignin after bleaching treatment. The viscoelastic properties of the molten biocomposites showed that HC addition to PLA matrix resulted in considerable increments in storage modulus, loss modulus and complex viscosity. The increments are attributed to the restricted molecular motions of PLA chains, hydrodynamical effects of PLA/filler interactions and creation of a 3D particulate network (Agustin-Salazar et al. 2018). On the fractured surface of PLA/HC biocomposites, some aggregates of separate HC particles in continuous PLA matrix were observed, but the fibers were completely impregnated with matrix and no fiber/matrix detachment was seen. Among all measured mechanical properties, HC filler, obtained from the NS bleaching process, just improved the flexural modulus of PLA to a considerable extent. In this work, the strength and strain at break of PLA/HC biocomposites determined by performing flexural stress-strain tests were equal to the corresponding results of PLA/untreated NS biocomposites. Nevertheless, the impact strength and resilience of the PLA-based biocomposites containing bleached NS were found to be even weaker than the related properties of pure PLA and PLA/untreated NS biocomposites. It was believed that it related to the lack of chemical bonding between bleached lignocellulosic filler and PLA matrix (Agustin-Salazar et al. 2018).

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5.5 Etherification Treatment Fiber treatment through the etherification reaction of accessible hydroxyl groups on the surfaces of lignocellulosic fibers leads to an improvement in the mechanical properties and performance of PLA/natural fiber biocomposites. Epoxide, alkyl halides and benzyl chloride are among the chemicals that can be used for etherification fiber treatment. Sodium hydroxide is also applied in etherification treatment reactions. In fact, NaOH plays an important role in the formation of charged intermediate species, which facilitate the nucleophilic addition of fiber surface groups to etherification treatment compounds like benzyl chloride (Avinc and Khoddami 2009). In some of the etherification treatment methods such as benzylation, wherein benzyl chloride is used as the treatment agent, the hydrophilic hydroxyl groups of lignocellulosic materials are substituted by more hydrophobic benzyloxy groups. That can reduce the fiber hydrophilicity. Unlike this method, which is limited to the chemical treatment of fiber surfaces, in epoxide-based etherification methods, the coupling reactions between lignocellulosic fibers and PLA matrix become possible.

5.5.1

Benzylation Reaction

Benzylation reaction, using benzyl chloride, is generally carried out in the presence of sodium hydroxide or after alkali pretreatment. As shown in Fig. 36, the benzylation reaction, also known as “Williamson synthesis”, is a nucleophilic reaction between alkoxide and halide ions. By substituting large and non-polar groups of benzyloxy for small and hydrophilic groups of hydroxyl, the intra- and inter-molecular hydrogen bonds among lignocellulosic constituents are reduced and correspondingly, the fiber

Fig. 36 Proposed reaction scheme of the benzylation treatment of lignocellulosic fibers after alkali pretreatment

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thermoplasticity is increased. Reaction time and temperature as well as media alkalinity are effective parameters in benzylation treatment process (Ghorbani Chaboki et al. 2019). In a research (Ghorbani Chaboki et al. 2019), rice straw fibers were chemically treated in aqueous solution of NaOH and benzyl chloride. In this study, the thermosetlike fibers after etherification process showed T g in the range of 170 °C, in dynamicmechanical thermal analysis (DMTA). Adding benzylated rice straw fibers (BRS) to PLA led to an augmentation in T g of PLA. Table 3 shows the increase in T g of matrix for different loadings of BRS, i.e. 10, 30 and 50 wt%. The reason behind this increment was believed to relate to improved interactions between thermopalsticized fibers and PLA matrix. The tensile stress-strain results showed that adding BRS fibers having less hydrophilicity and more thermoplasticity can simultaneously improve the Young modulus, yield stress, stress at break and elongation at break of PLA matrix at low fiber loadings (Fig. 37). Microscopic observations of the fractured surfaces of PLA/BRS biocomposites (Fig. 38) confirm the occurrence of fiber breakage as dominant phenomenon. At 10 wt% loading of BRS, less debonding at fiber/matrix interfaces is observable, which demonstrates the appropriate wettability of BRS fibers with PLA matrix. However, by increasing the reinforcing fiber content, intensified fiber pull-out and interfacial Table 3 Tg of virgin PLA, BRS and PLA/BRS biocomposites obtained from DMTA analysis, x in PLABRSx codes show the fiber loading (Ghorbani Chaboki et al. 2019)

Fig. 37 Stress-strain curves of virgin PLA and PLA/BRS biocomposites, x in PLABRSx codes show the fiber loading (Ghorbani Chaboki et al. 2019)

Sample

Tg (°C) at a frequency of 1 Hz

2 Hz

5 Hz

BRS

–

169

172

PLA

54.3

54.6

54.7

PLABRS10

56.10

57.24

58.78

PLABRS30

57.21

58.52

60.19

PLABRS50

60.36

61.16

62.58

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Fig. 38 SEM image of PLA/BRS biocomposites at a fiber loading of: a–b 10 wt%, c–d 30 wt% and e–f 50 wt% (Ghorbani Chaboki et al. 2019)

debonding eventuate in a brittle mechanical failure mode and elimination of yield point in stress-strain curves (Ghorbani Chaboki et al. 2019). In a similar study (Zandi et al. 2019), rice straw was first undergone alkali treatment with NaOH solution, and after separation of pulp and liquor via alkali pulping process, both products were benzylated. It can be seen in Fig. 39 that benzylated rice straw pulp has higher whiteness owing to the delignification of rice straw through soda pulping and benzylation reactions. The main component of rice straw pulp prepared from alkali treatment has been cellulose, while the main constituents of rice straw pulping liquor have been lignin and hemicellulose.

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Fig. 39 Scheme of alkali pulping and benzylation processes of rice straw (Zandi et al. 2019)

The SEM images of rice straw products after benzylation process are represented in Fig. 40. As can be seen, there are many pores at surface of benzylated liquor (BL) particles, while benzylated pulp (BP) consists of cellulosic microfibrils. The BL filler, which is more hydrophobic than cellulose, was found to show better interfacial adhesion with PLA matrix. Figure 41 confirms that even on the cryogenically-fractured surface of PLA/BL biocomposites, no sign of fiber pull-out and interfacial debonding is discernable. In Fig. 41c, the coverage of filler surface with a thin layer of matrix is obvious. These phenomena are attributed to the filler hydrophobicity increment after

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Fig. 40 SEM images of: a–b untreated rice straw, c–d BL and e–f BP (Zandi et al. 2019)

benzylation reaction and better mechanical interlocking with PLA matrix provided by the porosities on BL surfaces. Thermal analysis results showed that T m , T g and vicat softening point of PLA decreased by adding benzylated products of rice straw. As an exception, just the benzylated cellulose fibers at 10 wt% in PLA (PLABP10 composite) had higher softening point than virgin PLA. The resultant decrease of fiber crystallinity degree and increase of fiber thermoplasticity, despite improved interfacial adhesion, are the

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Fig. 41 SEM images of: a–b PLABL10, c–d PLABL20 and e–f PLABL30 biocomposites, x in PLABLx codes shows the filler loading (Zandi et al. 2019)

plausible explanations. Nevertheless, adding the benzylated products of rice straw to PLA matrix brought about a significant rise in matrix crystallinity degree, owing to the effective role of modified fibers as heterogeneous nucleation sites. The increment was observed to be more considerable for benzylated liquor product. Noticeable enhancements in the biodegradation rate of PLA in soil were also found by adding benzylated rice straw fillers. In a similar trend to crystallinity degree, PLA/BL biocomposites have shown higher biodegradation rate than PLA/BP biocomposites, at a constant filler loading (Zandi et al. 2019).

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Fig. 42 Opening reaction of epoxide rings by attacking a nucleophile

5.5.2

Epoxide Opening Reaction

Etherification treatment can be also occurred by epoxide opening reaction with a nucleophile attack in the presence of an acid (Fig. 42). One of the epoxides, used for the treatment of lignocellulosic fiber and improvement of natural fibers/PLA compatibility, is glycidyl methacrylate (GMA). GMA has been used for the treatment of rice straw fibers (RSF) in a study (Wu et al. 2013). In order to perform the GMA grafting reaction onto PLA chains, a mixture of GMA and benzoyl peroxide (BPO) was added to molten matrix. The reactions most likely proceed according to Fig. 43a to form GMA grafted PLA (PLA-g-GMA) chains. The fiber surface was treated by tetraethyl orthosilicate as crosslinking agent, H2 O and lactic acid as catalyst in tetrahydrofuran solution. During melt-mixing processes, the opening reaction of epoxide groups and PLA-g-GMA reaction with crosslinking agent grafted onto fiber surfaces became possible (Fig. 41b). With these reactions, lignocellulosic fibers covalently bond with PLA chains through the formation of several intermediate species. PLA-g-GMA/TRSF biocomposites, in comparison with PLA/RSF biocomposites, showed higher T g , melting enthalpy, thermal stability, tensile strength and HDT at fixed fiber loadings. However, the treated biocomposites had relatively smaller melt viscosity, water absorption and soil burial biodegradation rate. The SEM micrographs of fractured surfaces (Fig. 44) reveal the RSF pull-out, which stems from the high surface energy difference of RSF fiber with matrix phase. Better wettability of PLA-g-GMA matrix and higher strength of treated fiber/matrix interfaces led to more homogenous fiber distribution and complete coverage of treated fibers with matrix in such a way that no pull-out can be observable on Fig. 44b. Another notable point is the significant and unique increment in the tensile strength of PLA-g-GMA/TRSF biocomposites. Unlike untreated PLA/RSF biocomposites,

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Fig. 43 a GMA grafting reaction onto PLA chains and b the chemical between PLA-g-GMA and treated rice straw fiber (TRSF). Adapted to (Wu et al. 2013) Fig. 44 SEM image of fractured surface of: a PLA/RSF and b PLA-g-GMA/TRSF biocomposite at a constant fiber loading of 20 wt%. Reproduced with permission from (Wu et al. 2013)

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the tensile strength of treated PLA-g-GMA/TRSF biocomposites did not show a decline by increasing the fiber content (Wu et al. 2013). In a similar study (Liu et al. 2019), the interface of bamboo fibers and PLA matrix was modified using tannic acid (TA, with average 25 hydroxyl groups per molecule) and epoxidized soybean oil oligomers (ESO, with average 4 epoxy groups per molecule). To ensure a proper grafting reaction between the fibers and PLA chains, the fiber surfaces were first sprayed with ESO/TA solution in acetone. Afterward, the dried fibers were melt-compounded with PLA via twin-screw extruder. The utilized epoxide can act as a compatibilizer for fiber/PLA interfaces at high temperatures, through ring opening reaction and covalent bond formation with –OH groups of fibers along with the end groups of PLA chains. The reason of applying TA has been the localization of epoxy groups at the center of fatty acid (ESO) chains, through which their reactivity is limited owing to steric hindrance. Besides, TA can induce the ring opening reactions of epoxides, due to the presence of many phenolic –OH groups in the chemical structure. Through the fiber treatment, the phenolic –OH groups of TA first react with the epoxy groups of ESO, by which a layer of TA-ESO is formed. Then, in a second step, the epoxy groups of TA-ESO layer can concurrently react with the end groups of PLA and fiber hydroxyl groups. Furthermore, the TA-ESO layer can physically interact with fibers through strong and natural polyphenol-inspired chemistry like hydrogen bonds and π − π interactions (Fig. 45). For low concentrations of TA-ESO (0.5 wt%), tensile modulus and strength, elongation at break, tensile toughness, impact strength and storage modulus of untreated PLA/BF biocomposites were improved, because of the formation of chemical bonds

Fig. 45 Physical interactions and chemical linkages of TA-ESO with fiber surfaces and PLA chains. Adapted to (Liu et al. 2019)

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Fig. 46 a–c Mechanical characteristics of PLA/BF biocomposites versus TA-ESO concentration, d impact strength of untreated and treated PLA/BF biocomposites versus BF loading. Adapted to (Liu et al. 2019)

and TA-ESO flexible layer at the interfaces of PLA/lignocellulosic fiber composites. However, increasing the concentration of TA-ESO gradually changed the role of TA-ESO from a coupling agent to a toughening agent and only the properties of PLA/BF composites such as tensile toughness, impact strength and elongation at break improved at higher TA-ESO contents (Liu et al. 2019). Figure 46 illustrates the mentioned variations in properties versus TA-ESO concentration. In fact, the phase separation of TA-ESO phase, the formation of dispersed TA-ESO rubbery domains in PLA matrix and correspondingly, toughening of PLA matrix were reported as the explanations. In this chemical treatment, the improvement of the biocomposite properties is just possible by the formation of a thin TA-ESO layer at PLA/fiber interfaces, but in the case of high TA-ESO concentrations and creation of thick and weak interfacial layers of TA-ESO, the biocomposites easily undergo failure under stress. Hence, the concentration of epoxide coupling agent plays a key role in improving the biocomposite performance (Liu et al. 2019).

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5.6 Graft Copolymerization One of the most common topics on reinforced polymers is the treatment of lignocellulosic fibers through graft copolymerization. During this modification process, polymeric chains are synthesized in the presence of fibers and the chains are grafted onto fiber surface through covalent bonds. The polymerization and grafting reactions of polymer chains with fiber surfaces eventuate in the possibility of improving the physical, mechanical and biological properties as well as the surface characteristics and environmental responsiveness of fiber-reinforced composites (Zhang et al. 2017). Grafting proportion and efficiency control the compatibility of lignocellulosic fibers with polymer matrix. Moreover, the type and composition of polymerization initiator, co-monomer type and grafting reaction conditions are determinant parameters in the treatment process as well (Kozlowski and Wladyka-Przybylak 2004; Zimniewska and Wladyka-Przybylak 2016). Free radical polymerization, ionic and ring-opening polymerizations are different polymerization methods that can be employed in graft copolymerization of lignocellulosic fibers. The ionic polymerization, despite the possibility of further control of polymerization reaction, are not very common for this type of treatment. The explanation is related to the specific conditions required for such reactions and the possibility of side reactions on fiber surfaces during polymerization process (Zhang et al. 2017). On the other hand, free radical polymerization has gained more attention. The chain propagation reaction of radical polymerization can be initiated in different ways by using peroxides, oxidation-reduction agents, UV and γ radiation, cold plasma and etc. For instance, lignocellulosic fibers can be placed in an aqueous solution containing selected ions and exposed to high-energy radiation. As a result of radiation, the lignocellulosic molecules crack down and free radicals are formed. The fibers are then treated by a monomer that is compatible with polymer matrix and the polymerization reaction of monomers is initiated and propagated by the free radicals obtained from fibers. Different types of vinyl monomers, including acrylonitrile, styrene, methyl methacrylate and butyl acrylate, can be employed in this treatment method (Kozlowski and Wladyka-Przybylak 2004; Zimniewska and Wladyka-Przybylak 2016). The free radical polymerization methods such as reversible deactivation radical polymerization (RDRP), atom transfer radical polymerization (ATRP), reversible addition-fragmentation chain transfer polymerization (RAFT) and nitroxide-mediated radical polymerization (NMP) have been widely used in the treatment of lignocellulosic fibers. In addition, the possibility of ring-opening polymerization and grafting of the resultant polymer chains onto fiber surfaces exists, owing to the hydroxyl groups on lignocellulosic fiber surfaces. The cyclic monomers such as lactides and lactones can be used in ROP reaction and resulted in the synthesis of biodegradable PLLA and poly(E-caprolactone) (PCL) (Zhang et al. 2017). In related researches (Qin et al. 2011a, b), the surface of rice straw fibers was treated with butyl acrylate graft copolymerization. The treatment induced an improvement in the tensile strength of PLA up to 6 MPa, at constant weight

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percentage of 7.98 wt% for poly(butyl acrylate) (PBA) reacted with RSF. The graft copolymerization of butyl acrylate monomer in the presence of RSF, azobisisobutyronitrile (AIBN) initiator, poly(vinyl alcohol) (PVA) dispersant was performed in distillated water at a temperature of 75 °C. By grafting PBA onto the surfaces of lignocellulosic fibers, the water absorption of PLA/RSF biocomposites was decreased. Since PBA is relatively hydrophobic, the substitution of PBA chains for the hydrophilic hydroxyl groups of fibers and the resultant improvement of fiber/matrix interfacial interactions led to a decrease in the water diffusion of treated biocomposites. As a result of the reduction in the biocomposites hygroscopicity, a smaller drop in the biocomposite properties has been observed due to less water absorption. Moreover, PLA/treated RSF biocomposites show better thermal stability than PLA/untreated RSF biocomposites at a constant fiber loading. In this research, the SEM images of fractured surfaces of PLA/RSF biocomposites revealed large extent of fiber pull-out, whereas the fiber breakage was dominant phenomenon on the SEM micrographs of fractured surfaces of PLA/TRSF biocomposites. This indicates stronger interfacial bonding and emphasizes on the compatibility role of PBA coverage layer of modified fibers. However, using higher percentages of PBA resulted in weak interfaces and lower PLA/TRSF compatibility. The explanation is the poor distribution of grafted PBA chains, as a flexible polymer, in PLA matrix and the formation of a weak interface. At larger amount of PBA, the fiber surfaces are covered with a layer of stacked PBA, which is discernable on the SEM images of fractured surfaces of PLA/TRSF biocomposites. Therefore, the tensile strength of PLA/TRSF composites decreased by increasing PBA content after showing a maximum value, while strain at break followed an upward trend (Fig. 47). Moreover, it was observed that the heterogeneous nucleation effect of RSF

Fig. 47 Relationship between the tensile properties and additive rate of grafted PBA onto the fibers in PLA/RSF biocomposites. Adapted to (Qin et al. 2011b)

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surfaces for the PLA chain crystallization has been reduced by PBA grafting. The present PBA chains at fiber surfaces hinder the PLA chains from nucleating on fiber surfaces during the matrix crystallization process. Hence, XC of PLA in PLA/TRSF biocomposites diminishes and the cold crystallization temperature (TCC ) increases to higher values (Qin et al. 2011a, b). In a similar manner, the graft copolymerization of methyl methacrylate (MMA) monomer was also employed for the treatment of lignocellulosic fibers like RS and agave cellulose fibers in PLA matrix (Zhao et al. 2011; Rosli et al. 2019). It has been reported that poly(methyl methacrylate) (PMMA) has a good miscibility with PLA chains and only one glass transition temperature is observable in the DSC thermograms of PMMA/PLA blends. In one of the related researches (Zhao et al. 2011), MMA admicellar polymerization method was used to create a very thin polymer film on fiber surfaces. The “admicellar” word is derived from “adsorption” and “micelle” and means the monomer polymerization in a surfactant bilayer absorbed on the substrate surface. The surfactant concentration in these conditions must be less than the critical micelle concentration (CMC). Using this innovative method, the researchers first put RSF in cetylpyridinium chloride (CPC) solution to reach the equilibrium absorption of CPC molecules on fiber surfaces. Then, MMA monomer was added to the solution. After reaching the equilibrium adsolubilization of MMA monomer in admiclle, ammonium persulfate initiator was added to the solution and PMMA was polymerized at 70 °C. The fiber treatment with MMA grafting copolymerization was confirmed with FTIR spectroscopy results and microscopic observations. The microscopic observations demonstrate a large extent of fiber pull-out and interfacial voids for untreated fibers in PLA matrix. On the other hand, with the surface treatment and formation of strong bonds between grafted PMMA and PLA chains, the fiber breakage becomes the dominant phenomenon at fractured surfaces of PLA/RSF biocomposite. Even at higher magnifications, no obvious interface can be perceivable between PMMA-grafted fibers and PLA matrix. As can be seen in Fig. 48, the graft copolymerization treatment of RSF eventuates in an augmentation in the strain at break and tensile strength of PLA/RSF biocomposites. By the chemical treatment, the frictional stress at interfaces increases and correspondingly, the stress transfer efficiency improves. The slight decrease of tensile strength at larger amount of grafted PMMA in Fig. 48 relates to the extra grafted chains at interfaces, which brings about interfacial sliding of PLA and PMMA chains at smaller stresses (Zhao et al. 2011). The surface treatment of lignocellulosic fibers with graft copolymerization of ε-caprolactone via ROP polymerization was also applied for the improvement of properties of PLA/wheat straw fiber (WSF) biocomposites (Kellersztein et al. 2016). In this copolymerization method, the fibers were first treated with NaOH and then, benzyl alcohol as an initiator, ε-caprolactone monomer and tin octoate (SnOct2 ) as a catalyst were added to the solution. The ring-opening polymerization was carried out at 95 °C for 22 h. The proposed reaction mechanism of the PCL graft copolymerization on the surface of wheat straw fibers is shown in Fig. 49. The occurrence

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Fig. 48 Relationship between the tensile properties of PLA/RSF biocomposites and MMA content in the graft copolymerization reaction of fibers at a constant filler loading of 30% (v/v). Adapted to (Zhao et al. 2011)

of grafting reaction onto WSF was proven by proton nuclear magnetic resonance (1 HNMR) and attenuated total reflectance infrared spectroscopies (ATR). The addition of PCL-grafted WSF to matrix led to a decrease in the PLA crystallinity degree. It was believed that PCL-grafted chains hindered the crystallization process of PLA owing to the PCL/PLA interactions and entanglements. Additionally, the rheological measurements showed that the complex viscosity of PLA/WSF biocomposites was significantly increased by the graft copolymerization treatment of fibers. In fact, larger frictional forces between PCL-grafted fibers and matrix chains lead to an increase in viscosity. The chemical treatment also resulted in an improvement in tensile and flexural strength as well as strain at break of PLA/WSF biocomposites (Fig. 50). However, it must be noted that the incorporation of PCLtreated fibers into PLA matrix caused a slight decrease in tensile and flexural modulus compared to untreated fibers. It can be attributed to the augmentation of the amorphous part fraction of biocomposites, especially near PCL/PLA interfaces (Kellersztein et al. 2016).

5.7 Acid Hydrolysis Treatment One of the chemical treatment processes of lignocellulosic fibers is acid hydrolysis modification that is frequently applied after alkaline pretreatment. The acid hydrolysis treatment involves in degradation reactions, through which the average degree of polymerization of lignocellulosic polymers diminishes. Throughout the hydrolysis

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Fig. 49 Proposed reactions of ring-opening polymerization of ε-caprolactone with the surface of wheat straw fiber using benzyl alcohol as an initiator and tin octoate (SnOct2) as a catalyst. Adapted to (Kellersztein et al. 2016)

reaction of plant fibers in an acidic medium, the β-1,4-glucosidic bonds, which are present in the backbones of natural polymers such as cellulose and hemicellulose, or other etheric groups are split up by adding water molecules. By the reaction, shorter fragments from polymeric chains are formed, whereas the polymer basic structures are preserved (Fan et al. 1987). The reaction scheme of the acid hydrolysis reaction of cellulose is shown in Fig. 51. As mentioned, the acidic treatment of lignocellulosic materials can lead to the hydrolysis of lignin, pectin and hemicellulose molecules, thereby removing the amorphous components of plant substance from the cellulosic fibers. The direct result of that is a considerable reduction in the natural fiber diameter and an improvement in the fiber/matrix interfacial adhesion (Araújo et al. 2018). The acid hydrolysis treatment procedure was employed by Araújo et al. to improve the properties of PLA-based biocomposites containing eucalyptus and mallow fibers (Araújo et al. 2018). The hydrolysis reaction of mallow fibers was carried out after alkali pretreatment while the eucalyptus fibers were exposed to both alkali and bleaching pretreatments before the hydrolysis modification. The hydrolysis reaction was performed using sulfuric acid solutions at various concentrations for different treatment times.

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Fig. 50 Mechanical characteristics of virgin PLA and the biocomposites of PLA/untreated WSF, PLA/alkali-treated WSF and PLA/PCL-grafted WSF. Adapted to (Kellersztein et al. 2016)

As can be observed in Fig. 52, the bleached and mercerized eucalyptus fibers have smooth surfaces demonstrating the elimination of lignin and other impurities from the fibers through the pretreatment processes. The treated eucalyptus fibers (FE6 sample), hydrolyzed by using H2 SO4 solution at a concentration of 10% (v/v) at 80 °C for 240 min, preserved the primary fiber diameter. Despite that, the fiber length was reduced by the hydrolysis treatment. It seems that the hydrolysis reaction weakens the fibers and increases the surface roughness (Fig. 52c–e). The hydrolyzed eucalyptus fibers (FE7 sample) shown in Fig. 52f–h were treated by using sulfuric

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Fig. 51 Mechanism of the acid hydrolysis reaction of cellulose. Adapted to (Lelekakis et al. 2014)

acid solution at a concentration of 50% (v/v) at 60 ◦C for 240 min. As can be seen, the microfibrils were formed at the fiber ends by the chemical treatment. For the modified eucalyptus fibers (FE9 sample) with the SEM images of Fig. 52i–k, the treatment agent of H2 SO4 solution at a concentration of 30% (v/v) at 60 ◦C for 150 min was applied. These reaction conditions caused a drastic decrease in the primary fiber length and gave the FE9 fibers a rough and brittle appearance. Nonetheless, these hydrolyzed eucalyptus fibers did not show any noticeable changes in the diameter by the treatment (Araújo et al. 2018).

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Fig. 52 SEM images of: a, b untreated eucalyptus fibers, c–e FE6 fibers, f–h FE7 fibers and i–k FE9 fibers. Reproduced with permission from (Araújo et al. 2018)

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The XRD analysis confirms that the fiber crystallinity degree diminishes after acidic modification. By acid hydrolysis reaction of eucalyptus fibers, the interface of PLA matrix and reinforcing fibers becomes stronger and the efficiency of the interfacial stress transfer is heightened. The microscopic observations on the fractured surfaces of PLA/eucalyptus fiber biocomposites (Fig. 53) demonstrate that the tendency to fiber pull-out is reduced and less gaps can be discerned at matrix/filler

Fig. 53 SEM images of the fractured surface of PLA and PLA/eucalyptus fiber composites: a, b control sample, virgin PLA, c, d PLA/untreated fiber composite, e, f PLA/treated FE9 fiber composite. Reproduced with permission from (Araújo et al. 2018)

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interface by the acidic treatment. Furthermore, by improving the interfacial adhesion of PLA/acidic treated eucalyptus fiber phases, the storage modulus at ambient temperature, T g , PLA crystallinity degree, degradation onset temperature and temperature with maximum thermal degradation rate were raised to higher values comparing with untreated fiber-reinforced PLA composites at the same loading of natural filler (Araújo et al. 2018). In a similar work, the acid treatment method was used to reinforce the PLA matrix with randomly oriented short Spartium junceum L. fibers (Kovaˇcevi´c et al. 2019). For the preparation of fibers, in this work, first the fibers were treated by alkali solution, then were exposed to microwave radiation for 5 min to extract the fibers from plant stems. Different chemical modifications were applied including mercerization, alkali and nanoclay treatments as well as acidic and nanoclay treatment. The PLAbased biocomposites were prepared by the hand lay-up of fibers in mold and then, compression molding. The findings revealed that the alkali treatment decreased the fiber tenacity, whereas the other treatment methods, associated with nanoclay presence, resulted in enhanced fiber strength and tenacity. A comparison between these two methods, which involved in the use of nanoclay, clarifies the coarser surface of the acidic treated fibers (Fig. 54). Citric acid as the treatment agent in acidic modification process had stronger beneficial effects on treating the fiber surface and linking the montrmorillonite nanoparticles with fiber surfaces (Kovaˇcevi´c et al. 2019). The SEM images of the fractured surfaces of biocomposites shown in Fig. 54 demonstrate the differences in the interfacial strength of PLA matrix and the modified fibers. By alkali treatment, the extent of fiber/matrix debonding and fiber pull-out was reduced comparing with untreated fiber-reinforced PLA composite. In marked contrast, the biocomposite containing the treated fibers with NaOH and nanoclay showed a brittle mechanical behavior and debonding at the interface occurred to large extent. Comparatively, the PLA biocomposite, containing the lignocellulosic fibers modified by citric acid and nanoclay treatment agents, had smoother fractured surface and the fiber pull-out took place to lower extent. By improving the matrix/fiber interfacial strength and void reduction in the PLA-based composite containing treated fibers with citric acid and nanoclay, the highest mechanical properties such as elastic modulus, tensile strength, elongation at break and dissipated energy were attained. The column graph in Fig. 55 confirms the advantages of acidic and nanoclay treatment method in providing the biocomposite with better mechanical performance (Kovaˇcevi´c et al. 2019). According to the literature, the acid hydrolysis treatment has positive effect on the reinforcing role of plant fibers in PLA matrix. However, the improvements in final properties are controlled by the acid and fiber types, pretreatment reaction conditions, concentration of acidic solution and hydrolysis reaction time.

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Fig. 54 SEM images of the lignocellulosic fibers (left column) and the fractured surface of PLA/fiber biocomposites (right column): a untreated fiber, b PLA/untreated fiber, c treated fibers with NaOH, d PLA/treated fiber with NaOH, e treated fibers with NaOH and nanoclay, f PLA/treated fiber with NaOH and nanoclay, g treated fibers with citric acid and nanoclay, and h PLA/treated fiber with citric acid and nanoclay. (Kovaˇcevi´c et al. 2019) Open access

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Fig. 55 Mechanical characteristics of PLA and PLA/Spartium junceum L. fiber composites. Adapted to (Kovaˇcevi´c et al. 2019)

5.8 Isocyanate Treatment The isocyanate treatment of lignocellulosic fibers is carried out by using isocyanates or diisocyanates. Isocyanate treatment agents have –N=C=O functional groups, which are highly reactive against hydroxyl and carboxyl groups. Reportedly, isocyanates can play the common role of coupling agents in plant fiber-reinforced biocomposites (Siakeng et al. 2019a; Orue et al. 2018; Abu Ghalia and Dahman 2017). Isocyanate functional groups can react with the hydroxyl groups of cellulose, hemicellulose and lignin macromolecules and form urethane linkages. In Fig. 56, the reaction mechanism of urethane bond formation is shown. By the reaction, the fiber hydrophobicity and fiber resistance against moisture is enhanced. Besides, the interfacial strength between matrix and natural reinforcement agent is improved (Orue et al. 2018; Kozlowski and Wladyka-Przybylak 2004). The R atomic group in Fig. 56 can consist of different chemical groups like alkyls. The chemical bond formation between the isocyanate treatment agent and PLA chains will become possible, if the R group contains another reactive functional group. Under the circumstances, isocyanate molecules as a coupling agent can form chemical crosslinks between the matrix and lignocellulosic fiber phases (Luo et al. 2014). Despite that, the diisocyanate molecules having two –N=C=O functional groups can simultaneously react with the hydroxyl groups of plant fiber and hydroxyl or carboxyl end groups of PLA chains during the melt-compounding process of PLA/fiber biocomposites, without the need of a reactive functional group in R position. Therefore, the diisocyanate molecules can chemically crosslink the natural reinforcement agent with matrix

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Fig. 56 Reaction scheme of: a isocyanate molecule with lignocellulosic fiber, b diisocyanate molecule with lignocellulosic fiber and PLA chain

through the formation of urethane linkages (Fig. 56b) (Lee and Wang 2006; Yu et al. 2015). One of the isocyanate treatment agents, which was applied in a study for the chemical treatment of dry refined soft wood fibers in PLA matrix, has been octadecyl isocyanate (Krouit et al. 2010). By the substitution reaction of isocyanate groups associated with long alkyl groups for the fiber hydroxyl groups, the water contact angle increases from 39° for the pristine fibers to 104° for the isocyanate-treated fibers. Even though the treated lignocellulosic fibers become substantially more hydrophobic, slight changes in the glass transition temperature and mechanical properties of treated PLA/wood fiber composites were observed owing to the lack of chemical bonds between the reinforcing filler and PLA matrix (Krouit et al. 2010). In a similar work, isocyanate modified epoxy emulsion as a sizing material was employed to improve the properties of the PLA-based biocomposites containing alkali-treated corn fiber (ACF) (Luo et al. 2014). The used sizing material had lots of reactive groups such as epoxy and amino groups. These functional groups can concurrently undergo chemical reactions with the hydroxyl groups of fiber surfaces as well as the hydroxyl and carboxyl groups of PLA chains. Therefore, the sizing material can play the role of a coupling agent and enhance the fiber/matrix interfacial adhesion through the formation of intermolecular linkages between the reinforcing filler and polymeric matrix. In this work, it was found in the SEM micrographs of the fractured surfaces of tensile test samples that large cracks were present at the interfaces of PLA matrix and untreated corn fibers. The alkali treatment of the plant fibers eliminated interfacial voids and cracks, though the interfacial interactions

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between PLA matrix and ACF filler were not strong enough to totally suppress the fiber pull-out under stress transfer conditions. By sizing the alkali-treated fibers, not only no interfacial voids and cracks between the fiber and matrix were discernable, but also some stacked PLA residuals were observable on the fiber surfaces, indicating strong interactions between sized-ACF and PLA phases (Luo et al. 2014). The grafting function of isocyanate treatment agent, used in this work, resulted in considerable enhancements in the properties of PLA/untreated corn fiber (UCF) and PLA/ACF biocomposites (Luo et al. 2014). As can be seen in Fig. 57, the characteristics of corn fiber-reinforced PLA biocomposites including tensile modulus and strength, flexural modulus and strength, HDT, T g , T c and crystallinity degree were considerably improved by treating the fibers with the isocyanate sizing material. The used sizing chemical as an effective coupling agent markedly enhanced the corn fiber/PLA interfacial strength and diminished the molecular dynamics of PLA chains. However, the chemical crosslinking of PLA chains with natural fibers led to a reduction in impact strength and elongation at break of the biocomposites. In comparison to the isocyanate treatment agent with epoxy functional group in R atomic group (Fig. 56), the diisocyanate coupling agent has been reported to be more effectual to modify the PLA/lignocellulosic fiber interfaces and improve the properties of resultant biocomposites (Yu et al. 2015; Lee and Wang 2006). The grafting reaction of diisocyanate chemicals between PLA chains and plant fibers normally take place in melt-mixing processes in the presence of a catalyst. Some of the used diisocyanate treatment agents in PLA-based biocomposites have been isophorone diisocyanate (IPDI), 4,4 -diphenylmethane diisocyanate (MDI), 1,6hexane diisocyanate (HDI) and lysine-based diisocyanate (LDI). These chemicals as reactive compatibilizers, having two isocyanate groups, can simultaneously react with both plant fibers and PLA chains and form chemical crosslinks. The findings of related researches have clarified that the use of these reactive compatibilizers substantially improve different properties of PLA/ramie fiber composites including tensile strength and modulus, flexural strength, strain at break, impact strength, T g , T c , storage modulus, Tan δ peak position, thermal degradation temperature and flow activation energy (Yu et al. 2015; Lee and Wang 2006). Moreover, the effectiveness of diisocyanate coupling agent was reported to be dependent on the diisocyanate chemical structure and content. It can be seen in Fig. 58a and b that IPDI crosslinking agent is the most effective additive to improve the mechanical performance of PLA/ramie biocomposites, among different diisocyanate coupling agents used in this study including MDI, HDI and IPDI (Yu et al. 2015). In addition, the biocomposite properties against the coupling agent content show optimum values, which is evident in Fig. 58c and d. The incorporation of the diisocyanate compatibilizers into PLA/plant fiber composites can bring about considerable improvements in the properties just up to a fixed diisocyanate content and then, the properties level-off. At larger contents of the crosslinking agent, above a critical value, the diisocyanate molecules tend to aggregate and form a thick film on fiber surfaces. The thick film of coupling agent hinders the formation of uniform chemical crosslinks and effective fiber/matrix interactions (Yu et al. 2015; Lee and Wang 2006).

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Fig. 57 Different mechanical, thermal and thermo-mechanical characteristics of PLA/untreated corn fiber (UCF), PLA/alkali-treated corn fiber (ACF) and PLA/sizing-treated corn fiber (SCF) composites. Adapted to (Luo et al. 2014)

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Fig. 58 a and b Mechanical properties of PLA/ramie fiber biocomposites without/with different diisocyanate treatment agents, c and d Mechanical properties of PLA/ramie fiber biocomposites without/with IPDI coupling agent at different contents. Adapted to (Yu et al. 2015)

5.9 Peroxide Treatment Peroxides are chemical compounds with a general structure of R–O–O––R and a characteristic divalent O–O bond. At high temperatures, extremely reactive peroxide molecules decompose and form free radicals (Siakeng et al. 2019a; Kozlowski and Wladyka-Przybylak 2004). The free radical moieties can react with the natural polymers of lignocellulosic fibers like cellulose as well as polymer matrix chains and correspondingly, create some reactive sites on the macromolecules. As a result, the grafting reaction of matrix chains onto fiber surfaces at interfaces becomes possible by the mentioned free radical transfer reactions (Kozlowski and WladykaPrzybylak 2004; Orue et al. 2018). The proposed scheme of the reactions induced by peroxide treatment agents in PLA/natural fiber biocomposites is shown in Fig. 59. The frequently used organic peroxides in lignocellulosic fiber-reinforced composites are benzoyl peroxide, (C6 H5 CO)2 O2 , and dicumyl peroxide, (C6 H5 C(CH3 )2 O)2 . The peroxide treatment of plant fibers is normally carried out after alkali pretreatment (A. Orue et al. 2018). The grafting reactions of PLA chains onto fiber surfaces lead to an increment in the hydrophobicity of lignocellulosic fibers and matrix/fiber interfacial strength (Siakeng et al. 2019a). The peroxide chemical treatment associated with an alkali pretreatment

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Fig. 59 Proposed reactions induced by the peroxide treatment agent in lignocellulosic fiberreinforced PLA composites

process was used to improve the interfacial adhesion of the PLA/ramie fiber composites (Choi and Lee 2012). In comparison with unreinforced PLA, PLA/untreated fiber, PLA/alkali-treated fiber and PLA/alkali & silane-treated fiber samples, the PLAbased biocomposites, containing the ramie fibers treated with NaOH and peroxide chemicals, had the lowest thermal expansion in this work. The thermal expansion of biocomposites is closely related to interfacial properties. The presence of cracks, voids and striations at fiber/matrix interfaces brings about a large thermal expansion. The lowest thermal expansion of PLA/peroxide-treated fibers composite is provoked by the grafting reactions of free radical moieties, which result in the PLA chain grafting onto fiber surfaces and conclusively, better interfacial performance. Better matrix/fiber interactions through the formation of covalent linkages caused the largest impact strength for prepared PLA/ramie fiber composites having the plant fibers modified by dicumyl peroxide and NaOH compounds. Despite that, the measurements of the mechanical properties of a single fiber revealed that the peroxide treatment reactions have had a slight impact on the fiber strength and modulus itself (Choi and Lee 2012).

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Furthermore, the tensile strength and modulus of the biocomposites containing peroxide-treated fibers were determined to be lower than the ones for PLA/untreated fiber and PLA/alkali-treated fiber composites, at the same filler loadings (Fig. 60). The plausible reasons were believed to be the poor dispersion state of treated fibers and remaining intact fiber bundles in PLA matrix. Although the peroxide chemical

Fig. 60 a Tensile strength and modulus of treated and untreated single ramie fiber, b tensile properties and notched Izod impact strength of PLA and PLA/ramie fiber composites containing untreated and treated fibers. Adapted to (Choi and Lee 2012)

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treatment is noticeably effective in improving the fiber/matrix interfacial adhesion, the final performance of plant fiber-reinforced composites is also controlled by the composite preparation processes. In the reviewed research, the preparation processes consisted of physical mixing of PLA fibers with ramie fibers and then, compression molding (Choi and Lee 2012). In contrast to this work, in another research, benzoyl peroxide was applied to modify the combined banana/sisal fibers and the PLA-based biocomposites were prepared through twin-screw extruder and injection molding processes (Asaithambi et al. 2014). Over the melt-compounding process, the organic peroxide created highly reactive free radicals, which successfully induced the PLA chain grafting reaction onto the mercerized fiber surfaces. By applying the efficient melt-compounding process and proper function of peroxide treatment agent, the interfacial interactions, wettability and compatibility of PLA chains with the alkali-treated fibers were significantly increased. In these circumstances, the dominant phenomena on the fractured surfaces of PLA/peroxide-treated fibers composites were observed to be fiber fracture and matrix deformation. Nevertheless, fiber aggregates, fiber pull-out and interfacial cavities were greatly evident on the fractured surface of prepared PLA/untreated fibers biocomposites. As can be found in Fig. 61, peroxide chemical treatment prompts considerable improvements in the mechanical characteristics of PLA/sisal and banana fiber composites such as tensile strength and modulus, flexural strength and modulus and impact strength. By inducing strong PLA/fiber interactions through the formation of covalent linkages, the load transfers from matrix to reinforcing fibers with higher efficiencies (Asaithambi et al. 2014). According to the results, it can be concluded that the properties of the PLA-based biocomposites containing peroxide-treated lignocellulosic fibers are controlled by the reaction conditions of alkali pretreatment and peroxide treatment processes such as peroxide solution concentration and reaction time and temperature along with the melt-mixing and shaping process conditions.

5.10 Permanganate Treatment Permanganate treatment of natural fibers is generally performed by using potassium permanganate solutions in acetone. Through this chemical treatment, the extremely reactive permanganate ions react with the hydroxyl groups of lignocellulosic fibers (Siakeng et al. 2019a). According to the reaction mechanism shown in Fig. 62, MnO-4 ions induce grafting copolymerization between PLA chains and plant fibers by forming reactive sites on fiber surfaces. By the permanganate treatment, the hydroxyl groups of fibers are oxidized to carbonyl and carboxyl functional groups and the heteroatom dissociation of carbon-carbon and carbon-oxygen bonds occurs. The created active sites on fibers can provoke the grafting chemical reactions with PLA chains (Zou et al. 2012). Therefore, the resultant chemical interlocking at fiber/matrix interfaces is significantly enhanced the interfacial strength (Orue et al.

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Fig. 61 Mechanical characteristics of virgin PLA, PLA/untreated sisal/banana fibers composites and PLA/peroxide-treated sisal/banana fibers composites. Adapted to (Asaithambi et al. 2014)

2018; Kozlowski and Wladyka-Przybylak 2004). Additionally, the active permanganate ions can also react with the hydroxyl groups of non-cellulosic components of lignocellulosic fibers like lignin and hemicellulose. The occurrence of reactions with non-cellulosic impurities makes the fiber surfaces rougher and fiber/matrix interfaces stronger by providing mechanical interlocking (Zou et al. 2012). In addition to the created mechanical and chemical interlocking, the fiber hydrophilicity is reduced by the graft copolymerization and oxidation reactions induced by permanganate treatment agent (Siakeng et al. 2019a). The influences of permanganate treatment process on lignocellulosic fibers were investigated on banana fibers (Paul et al. 2010). In this study, banana fibers were

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Fig. 62 Probable reactions mechanism involved in the permanganate chemical treatment of lignocellulosic fibers

soaked in 0.5% permanganate solution in acetone for 90 min after alkali pretreatment. The microscopic observations of treated fibers demonstrate an increment in fiber roughness, which can potentially result in stronger matrix/filler interfaces through mechanical interlocking. Moreover, the authors reported that permanganate treatment decreased the acidity and polarity of fibers and adversely, increased the wettability of treated fiber with polymeric matrices (Paul et al. 2010). However, the efficiency of permanganate treatment method to improve the performance of PLA/natural fiber biocomposites is dependent on the treatment solution concentration, fiber soaking time and alkali pretreatment (Orue et al. 2018). The impacts of permanganate treatment variables on the moisture content as well as structural and mechanical properties of abaca fibers have been also studied (Batara et al. 2019). In this research, it was found that abaca fibers, treated with potassium permanganate solution at concentration of 0.125% for soaking time of 3 min without exposing to mercerization pretreatment, had the highest tensile strength and lowest moisture content. Besides, permanganate modification aroused in rougher and more hydrophobic fibers capable of providing stronger mechanical interlocking. However, an increment in the fiber soaking time, up to 20 min, and the concentration of potassium permanganate solution, up to 0.5%, as well as the fiber exposure to alkali pretreatment were found to damage the fibers and reduce the fiber strength. In severe reaction conditions, the fibrils are separated from each other, fiber bundles are opened and strength of hydrogen bonds inside lignocellulosic fiber diminish. Consequently, the fiber strength worsens. Since the properties of plant fiber-reinforced composites are greatly affected by the performance of reinforcing fibers, a reduction in the fiber strength itself can adversely impact the reinforcement role of fibers in matrix phase (Batara et al. 2019). The influences of permanganate chemical modification of natural fibers in PLA matrix were evaluated in a study by using short sisal fibers (SSF) and 0.1% KMnO4 solution in acetone (Zou et al. 2012). As can be observed in Fig. 63, permanganate treatment has made the surfaces of sisal fiber (SF) rougher, which can lead to the matrix/dispersed phase mechanical interlocking in PLA/SF biocomposites. In

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Fig. 63 SEM image of: a untreated short sisal fiber, b short sisal fiber treated with permanganate solution, c fractured surface of PLA/untreated sisal fiber sample and d fractured surface of PLA/permanganate-treated sisal fiber sample. Reproduced with permission from (Zou et al. 2012)

fact, the stronger interfacial adhesion of permanganate-treated fibers in PLA matrix induces lower extents of fiber pull-out and interfacial void formation on the fractured surfaces, which is obvious in the SEM images of Fig. 63. Furthermore, it can be discerned in the SEM image that permanganate-treated fibers are tightly connected to PLA matrix (Zou et al. 2012). By the permanganate treatment of sisal fibers and improvement of interfacial strength, Young modulus and tensile and impact strength of PLA/untreated SSF biocomposites were increased. As can be found in Fig. 64, the reinforcement influences of permanganate-treated SSF are more considerable than the ones for untreated SSF. Nonetheless, among different PLA/treated fiber biocomposites prepared in this work, the biocomposite containing permanganate treated SSF has the lowest tensile and impact strength (Fig. 64). The other chemical treatment methods including acetylation, silane and alkali modifications have been more effective than permanganate treatment in improving the strength of matrix/fiber interface as well as PLA/SSF biocomposite (Zou et al. 2012). In a similar study, permanganate treatment method was employed to modify unidirectional jute fibers after performing mercerization pretreatment (Goriparthi et al. 2012). The alkali-treated fibers, in this work, were dipped in 0.125% permanganate solution in acetone for 1 min. Additionally, other chemical modifications including silane and peroxide methods were also applied. The results showed that different

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Fig. 64 Mechanical characteristics of virgin PLA, PLA/untreated SSF composite and PLA/treated SSF composites. Different chemical treatment methods were applied to modify the fibers including alkali, acetylation, silane and permanganate procedures. Adapted to (Zou et al. 2012)

properties of PLA/untreated jute fiber and PLA/alkali-treated jute fiber composites were improved by treating the plant fibers with permanganate solution. The mentioned properties include tensile modulus and strength, flexural modulus and strength, storage modulus, glass transition temperature and abrasive wear resistance. In spite of that, the properties of PLA/jute fiber composites containing fibers treated with silane and peroxide methods were better than the ones for PLA-based composite having permanganate–treated fibers (Goriparthi et al. 2012).

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5.11 Acrylation Treatment The acrylation modification is another chemical treatment method that can induce graft copolymerization in natural fiber-reinforced composites. This treatment involves in using acrylics and acrylonitrile monomers. The acrylic monomers can react with lignocellulosic fiber surface through two different reaction paths, (I) free radical reaction and (II) esterification reaction. Dependent on the matrix type used to produce the biocomposite, each one of the reaction paths can be used to modify plant fibers. In the first treatment path, lignocellulosic fibers are exposed to high-energy radiations, hence undergoing chain scission reactions and free radical formation (Orue et al. 2018). Acrylic monomers like acrylic acid are then grafted onto the resultant fiber macroradicals and acrylic monomer polymerization continues through free radical chain polymerization. In contrast, in the second treatment path, acrylic acid monomers react with the fiber hydroxyl groups through esterification reaction. By that reaction, acrylic monomers are grafted onto fiber surfaces and the free radical polymerization of vinyl monomers is performed by adding thermal initiators like organic peroxides. Because of the free radical polymerization of acrylic monomers, it is possible to create chemical linkages between polymeric matrix and lignocellulosic fibers. By the graft copolymerization of acrylic vinyl monomers, treated plant fibers become more hydrophobic and the fiber resistance against moisture improves. Stronger interfacial interactions between lignocellulosic fiber and matrix are provided by acrylation treatment as well (Orue et al. 2018; Siakeng et al. 2019a). The mechanisms of possible chemical reactions involved in the treatment of lignocellulosic fibers with acrylic monomers are shown below (Fig. 65). In a study, acrylic acid monomer was used to treat sunn hemp fibers (Kalia and Kumar 2013). The fibers were exposed first to NaOH solution, then acrylic acid solution. The appearance of an additional absorption peak in the FTIR spectrum of acrylated fibers at a wavenumber of 1733 cm−1 confirmed the presence of carbonyl group of acrylic acid monomer on fiber surfaces. Authors reported that the vinyl polymer was successfully deposited on fiber surfaces, which made the fibers rougher. Contrary

Fig. 65 Probable chemical reactions involved in the treatment of lignocellulosic fibers with acrylic monomers

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to the other used chemical modifications, namely peroxide and permanganate treatments, the acrylation method resulted in the fibers with higher thermal stability and crystallinity degree (Kalia and Kumar 2013). Similar to acrylic monomers, acrylonitrile monomer can be also grafted onto fiber surfaces, polymerized and chemically modified the natural fibers. Acrylonitrile monomer having free radicals can react with lignocellulosic macromolecules through dehydrogenation and oxidation reactions. In addition, the active free radical sites in the system can also react with matrix chains. Therefore, the efficiency of matrix/fiber interlocking is increased by the formation of chemical linkages between matrix chains and plant fibers (Orue et al. 2018). Reportedly, the grafting reaction between cellulose fibers and acrylonitrile monomer can favorably progress without lignin presence (Kozlowski and Wladyka-Przybylak 2004). The acrylonitrile chemical treatment was applied in a research to modify nonwoven jute fiber mat (Khan et al. 2015). To treat the fibers in this work, jute fibers were bleached by using NaClO2 solution. Then, the jute bleached fibers were exposed to acrylonitrile monomer in the presence of K2 S2 O8 initiator and Fe2 SO4 catalyst. Afterwards, non-woven mats from the treated and untreated fibers were prepared and the PLA-based biocomposites were obtained by using a hot press-molding machine. In this research, the jute fiber mats were also treated by other chemical treatment methods including alkali, bleaching, alkali-acetylation, alkali-permanganate, alkalibenzoylation and bleaching-isocyanate modifications. As can be seen in Fig. 66, just similar to other applied chemical treatments, the jute fiber mats treated by bleaching-acrylonitrile grafting reactions reinforce PLA matrix more effectively comparing to untreated jute mats. By acrylonitrile grafting treatment, the tensile, flexural and impact strength of PLA/bleached jute mat biocomposites improved by 13.5, 7.7 and 10%, respectively. Moreover, the water uptake of PLA/fiber composites containing bleached-acrylonitrile grafted jute mats had been also lower than other corresponding samples. The observed improvements in the properties of these composites are attributed to better compatibility of PLA matrix and jute mats, which stems from the formation of vinyl polymer layer on fiber surfaces and the resultant chemical interlocking (Khan et al. 2015). By the chemical treatments of jute fibers, matrix/fiber interfacial adhesion was improved considerably. The improvement significantly decreased the extent of interfacial debonding and fiber pull-out on the fractured surfaces. Figure 67 shows the SEM images of the fractured surfaces of prepared biocomposites. As can be observed, the acrylonitrile grafting treatment enhances the matrix/fiber interfacial strength and reduces the extent of fiber pull-out remarkably (Khan et al. 2015).

5.12 Modifications Based on Eco-Friendly Chemicals Among all different modification methods of lignocellulosic fibers, the chemical treatments are the most frequently used methods. This is due to their relative simplicity, low cost and/or high efficiency (Orue et al. 2018; Asha et al. 2017).

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Fig. 66 Mechanical characteristics of PLA/non-woven jute mats composites containing untreated and treated fibers using different chemical modification methods. Adapted to (Khan et al. 2015)

Although many methods of chemical modifications are effective to some extent in improving the properties of natural fiber-reinforced biocomposites, their ecofriendly nature is controversial (Church et al. 2015). In the synthesis of “green” PLA/lignocellulosic fiber composites, the use of eco-friendly and environmentally sustainable methods for the preparation of biocomposites are very important. Many of the mentioned chemical methods involve in the use of toxic chemicals, which are harmful to environment and operator. For example, the used sodium hydroxide in mercerization method is very corrosive, or isocyanates are very toxic and carcinogenic. In addition, some of these methods consist of multi-step reactions that are not energetically efficient (Church et al. 2015; Chaitanya and Singh 2018a). Therefore, these problems affect the environmental acceptability and commercial viability of the treated plant fiber-reinforced biocomposites (Chaitanya and Singh 2018a). As a result, the chemical treatment methods using more eco-friendly and non-toxic

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Fig. 67 SEM image of the fractured surface of a virgin PLA and PLA/non-woven jute mat composite containing: b untreated fibers, c bleached fibers, d bleached-acrylonitrile grafted fibers, e bleached-isocyanate treated fibers, f alkali-treated fibers, g alkali-acetylated fibers, h alkaliKMnO4 treated fibers and i alkali-benzoylated fibers. Reproduced with permission from (Khan et al. 2015)

chemical substances, and physical and physiochemical methods along with biological methods are considered as promising alternatives for common chemical methods (Orue et al. 2018). In order to overcome the problem of interfacial incompatibility in PLA-based biocomposites containing lignocellulosic fibers, the chemical methods using more eco-friendly materials have been developed, instead of complex and harmful chemical processes. In one of the related works, the absorption of amphiphilic block copolymers based on PEG onto fiber surfaces was employed to improve the compatibility of PLA matrix with cellulose fabric, obtained from bamboo fibers (Church et al. 2015). The used copolymers were PEG-block-oleyl ether copolymers (PEGn – b–C18 ), which were composed of hydrophilic and hydrophobic amphiphiles and had hybrid function. In the copolymer structures, n refers to the number of PEG units that was 2, 10 and 20 for the copolymers used in this research. These lengths of PEG blocks resulted in the hydrophilic-lipophilic balance (HLB) values of 4, 12 and 15, respectively. For deposition of the copolymers on cellulose fiber surfaces, simple dip-coating method was used for short periods of time. In this treatment method, water or water/acetone solutions of copolymers were applied to obtain micro-crystal suspensions or amphiphilic emulsions. Considering the used treatment agents, this method is superior to common chemical methods in terms of environmental concerns.

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By localizing the amphiphilic block copolymers at interface, strong interactions such as inter- and intra-molecular hydrogen bonding emerge across the interfaces of cellulose fibers and PLA matrix. In fact, PEG blocks are extremely flexible and hydrophilic and can form strong hydrogen bonds with cellulose surfaces. Figure 68 demonstrates that the deposition of PEGn –b–C18 copolymers on cellulose fiber surfaces improves the flexural modulus and strength. The highest reinforcing effect belongs to the cellulose fabric treated with PEG2 –b–C18 copolymer. PEG2 –b–C18 copolymer having smaller HLB, comparing to the copolymers with longer PEG chains, is more hydrophobic. The short PEG blocks result in less fiber slipping and lower degrees of dissociation and restructuring of hydrogen bonding at PEG/cellulose interface in stress transfer conditions. Comparing to the other prepared biocomposites, the fiber treatment with PEG2 –b–C18 copolymer caused the largest storage modulus, least damping and maximum shift of T g to higher temperatures. In other words, the application of this amphiphilic copolymer restrains the molecular motions at interface more considerably and improves the interfacial adhesion of reinforced fabric and matrix more (Church et al. 2015). More eco-friendly chemical materials, capable of forming chemical linkages, have been also applied to modify PLA/natural fiber biocomposites. In a research, aqueous solution of sodium tetraborate decahydrate (borax) was used to enhance the interfacial bonding between PLA matrix and different lignocellulosic fibers (BirninYauri et al. 2016). The used plant fibers were kenaf core fibers (KCF), oil palm empty fruit bunch fibers (EFBF) and oil palm mesocarp fibers (OPMF). Borax treatment has been suggested as a substitute for mercerization method, since boric acid and borax are less harmful in comparison with NaOH. Borax is relatively eco-friendly

Fig. 68 Flexural modulus and strength of PLA/cellulose fabric biocomposites, effects of the surface treatment of the fabric with PEGn-b-C18 copolymers. Adapted to (Church et al. 2015)

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and non-toxic. The borax treatment was found to increase the cellulose content of plant fibers and remove hemicelluloses and waxes from fiber surfaces. Moreover, the lignin concentration was decreased slightly at the surface by the treatment. As it can be seen in FTIR spectra in Fig. 69, the fiber treatment caused the disappearance of the absorption peak at 1718–1725 cm−1 , which is attributed to the C=O bond of carbonyl groups in hemicellulose and lignin. Moreover, the intensity of absorption peak related to the stretching vibration of aromatic C=C bonds in lignin at 1610– 1619 cm−1 decreased indicating the slight removal of lignin after borax treatment. The absorption peak at 1238–1239 cm−1 , corresponds to the stretching vibrations of etheric, phenolic and alcoholic C–O bonds, shifted to smaller wavenumbers or totally removed by the treatment. Overall, these results can confirm the elimination

Fig. 69 FTIR spectra of plant fibers with and without borax treatment. The BX letters in front of the fiber names show the use of borax treatment. Reproduced with permission from (Birnin-Yauri et al. 2016)

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Fig. 70 Proposed chemical reactions involved in the plant fiber treatment with aqueous solution of sodium tetraborate decahydrate, including the reactions of borate ion formation and the reactions of the resultant ions with fiber hydroxyl groups. Adapted to (Birnin-Yauri et al. 2016)

of lignin and waxy constituents from fiber surfaces (Birnin-Yauri et al. 2016). The probable reactions involved in borax treatment are shown in Fig. 70. The SEM images of the used fibers, in this work, before and after borax treatment have shown that borax-treated EFBF has rough and textured surface. The surface of treated OPMF has relatively more pores, which can assist mechanical interlocking of fiber/matrix phases. For KCF also, clean and rough surface was obtained by the treatment. In fact, borax treatment led to the removal of fiber surface impurities and created stronger fiber/matrix bonding. Therefore, impact, tensile and flexural strength along with flexural modulus and elongation at break of the PLA/plant fiber biocomposites improved via this chemical treatment. Furthermore, the PLA-based biocomposites containing borax-treated KCF, as a secondary fiber at 5 wt% loading, in combination with borax-treated EFBF or OPMF, as a primary fiber at 55 wt% loading, showed a reduction in water uptake and thickness swelling (Fig. 71). The lower hygroscopicity of PLA/borax-treated fiber biocomposites can enhances the composite performance in long term (Birnin-Yauri et al. 2016). Sodium bicarbonate has been also used as an eco-friendly substitute for ecologically hazardous sodium hydroxide (Chaitanya and Singh 2018a, b). Sodium bicarbonate or baking soda is widely applied in cooking, detergents and biomedical applications such as toothpastes and anti-acid agents. Moreover, this chemical has been used as a biopesticide with no reported negative effect on environment. Baking soda has been employed for the chemical treatment of sisal and Aloe Vera fibers (Chaitanya and Singh 2018a, b). Sodium bicarbonate plays a similar role to sodium hydroxide in the treatment process of fibers (Fig. 72), except that it has milder alkali nature. On that account, longer exposure time of plant fibers in sodium bicarbonate solution is required to achieve the desired results, compared to the treatment in NaOH solutions. The sodium bicarbonate treatment, similar to alkali treatment with NaOH solutions, removes parenchyma cells and impurities from fiber surfaces. Figure 73 shows

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Fig. 71 Dimensional stability of PLA/combined lignocellulosic fibers biocomposites containing untreated and borax-treated fibers. Reproduced with permission from (Birnin-Yauri et al. 2016)

Fig. 72 Proposed chemical reactions involved in the sodium bicarbonate treatment of lignocellulosic fibers

that after 24 h of soaking process, these cells were removed from the fiber surfaces, but no traces of fibrillation can be observable. After 72 h of exposure, fiber fibrillation began and fiber surfaces become rough. Hemicellulose and other impurities were removed from fiber surfaces at this time. At these circumstances, strong hydrogen bonds among inter-fibrillar areas packed the fibers closely. The rearrangement of fibers brought about more fiber homogeneity and stronger load bearing capability. For longer soaking time, more fiber bundles were broken and smaller fibrils with smooth surfaces were obtained (Chaitanya and Singh 2018b). The changes in the composition of lignocellulosic components of raw Aloe Vera fibers (RAF) after sodium bicarbonate modification at different soaking times (x in TAF-x code) can be found in Fig. 74a. Tensile, flexural and compression strength as well as tensile and flexural modulus of PLA/Aloe Vera fiber biocomposites after

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Fig. 73 SEM micrographs of untreated and treated Aloe Vera fibers (TAF), Sodium bicarbonate treatment was used at different soaking times. The soaking time is shown by x in TAF-x codes on the images. Reproduced with permission from (Chaitanya and Singh 2018b)

72 h of soaking time in sodium bicarbonate solution reached the maximum levels, as a result of changing the composition of fiber constituents (Fig. 74b–d). However, increasing the exposure time over 72 h led to weaker fibers with more intensified fibrillation and smoother fiber surfaces. Accordingly, the smoother surfaces result in poor matrix/fiber mechanical interlocking and intensified fibrillation causes more stress concentration at the fiber ends, which increase per unit volume by fibrillation phenomenon. Thus, the mechanical properties of PLA/treated RAF biocomposites were weakened by increasing the treatment times over 72 h, which can be seen in Fig. 74b–d. Unlike the mentioned mechanical properties, the maximum impact

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Fig. 74 a Composition of lignocellulosic constituents of untreated and treated Aloe Vera fibers (TAF), b–d mechanical characteristics of virgin PLA and PLA/Aloe Vera fiber biocomposites containing untreated and treated fibers. Sodium bicarbonate treatment was used at different soaking times (x in TAF-x codes relates to soaking time). Adapted to (Chaitanya and Singh 2018b)

strength was achieved at 48 h of fiber soaking time. This goes to lower extent of fiber pull-out and consequent lower dissipated energy at longer treatment time. As can be observed in Fig. 75, some microvoids are present on the fractured surface of PLA-based composites containing untreated fibers. Microvoids refer to the points of stress concentration that facilitate the crack propagation. By sodium bicarbonate treatment, fiber/matrix bonding is improved, fiber pull-out is impeded and fiber breakage becomes the dominant phenomenon at the fractured surfaces. However, the improvements were found to be more pronounced up to a certain limit of fiber soaking time (Chaitanya and Singh 2018b). The literature review reveal that it is possible to improve the properties of PLA/natural fiber biocomposites by using nontoxic and eco-friendly chemicals, which can effectively enhance the compatibility of fiber/matrix phases.

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Fig. 75 SEM image of tensile fractured surface of PLA/Aloe Vera fiber biocomposites containing untreated and treated fibers. Sodium bicarbonate treatment was used at different soaking times (x in TAF-x codes relates to soaking time). Reproduced with permission from (Chaitanya and Singh 2018b)

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6 Conclusion and Future Perspective Green composites that consist of lignocellulosic fibers as reinforcing filler and biopolymers as matrices have many beneficial advantages such as availability, renewability, eco-friendly, biodegradability and low density. These biocomposites have been introduced as the most promising alternatives for the products based on synthetic polymers. The plant fiber-reinforced poly (lactic acid) (PLA) biocomposites are the most widely used green composites among others. Although PLA/lignocellulosic fiber biocomposites are sustainable and ecologically friendly, the interface of PLA matrix and plant fibers must be modified to achieve a composite having comparable properties to petroleum-based composites. Different treatment methods including chemical, physical, physio-chemical and biological modifications have been developed to enhance the interfacial strength of matrix/lignocellulosic fibers and correspondingly, the biocomposite final properties. Comparing to other methods, the chemical modifications are the most frequently used procedures. Reviewing the literature, related to the PLA/lignocellulosic fibers systems, reveals the beneficial advantages of chemical treatments such as relative simplicity, lower cost and/or high efficiency. Different works on PLA/plant fiber biocomposites have shown that the chemical methods can effectively improve the biocomposite properties. The reasons behind that are the increments in fiber roughness and hydrophobicity, improvements in fiber/matrix interfacial adhesion through mechanical interlocking and chemical bonding and removal of impurities from fiber surfaces by performing the chemical treatments. However, the effectiveness of chemical methods depends on the proper choice of reaction conditions. There are some influential variables in chemical treatments that control the extent of improvements. The type of chemical treatment agent, reaction time and temperature, concentration of chemical agent in treatment solutions, pretreatment methods and applied processes for fiber/matrix compounding and shaping are of significance to improve the fiber/matrix interfacial adhesion and biocomposite performance. Reportedly, it has been found that the inappropriate choice of the processing factors involved in chemical treatment methods can lead to even the biocomposite properties deterioration. As a notable point, the eco-friendly nature of many chemical treatment methods is controversial. Many of chemical methods involve in the use of toxic chemicals, which are harmful to environment and operator. For instance, the used sodium hydroxide in mercerization method is very corrosive, or isocyanates are very toxic and carcinogenic. Therefore, these problems affect the environmental acceptability and commercial viability of the plant fiber-reinforced biocomposites treated by chemical reactions. As a result, some chemical treatment methods using more ecofriendly and non-toxic chemical substances such as sodium bicarbonate have been considered as promising substitutions for common chemical methods. Therefore, the use of eco-friendly and non-toxic chemical compounds will provide treated plant fiber/biopolymer composites with comparable properties to petroleum-based polymers and composites.

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3D Printing of Continuous Natural Fibre Reinforced Biocomposites for Structural Applications A. Mugeshwaran, Nalini Ranganathan, R. Joseph Bensingh, and Sanjay K. Nayak

Abstract In last few years, there is a constant demand from several industrial fields for using light-weight components to exhibit high mechanical performance. This demand is met by employing composite materials, especially the fiber reinforced polymer composites. Compare to conventional process shape complexity, infill density, and manufacturing lead times are no longer barriers with additive manufacturing. Therefore, the fabrication of light weight fiber reinforced polymeric composites using additive manufacturing remains the protagonist. In this chapter different types of fiber reinforce composites developed using different additive manufacturing technique are classified based on the length of fiber, aspect ratio, orientation and performance i.e. short, long and continuous fiber reinforced composites. Further the type of continuous natural fiber reinforced composites that are developed using various additive manufacturing technique such as fused deposition modelling (FDM) or Fused Filament Fabrication (FFF), stereolithography (SLA), selective laser sintering (SLS), selective laser melting (SLM), and Direct ink writing (DIW) were reviewed and discussed. In addition, different materials, drawbacks, and strengths associated with different additive manufacturing processes were detailed. Few examples were also presented in the chapter were the 3D Printed structural components has its own real time application in various manufacturing sectors namely automotive, aerospace and aviation. Keywords Additive manufacturing · Natural fiber · Reinforcement · Polymeric matrix

A. Mugeshwaran · N. Ranganathan (B) · R. J. Bensingh SARP: Advanced Research School for Technology and Product Simulation, TVK Industrial Estate, Guindy, Chennai, India e-mail: [email protected]; [email protected] S. K. Nayak Central Institute of Plastics Engineering and Technology (CIPET), TVK Industrial Estate, Guindy, Chennai, India © Springer Nature Singapore Pte Ltd. 2021 M. T. Hameed Sultan et al. (eds.), Biocomposite Materials, Composites Science and Technology, https://doi.org/10.1007/978-981-33-4091-6_6

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1 Introduction Continuous Natural Fiber (CNF) and their biocomposite are being widely used in almost all the structural application due to its massive natural characteristics like lower density, lower cost, and they are renewable and biodegradable with the environment (Pickering et al. 2016). “Green” composites, based on natural fibers derived from plants and biodegradable resins, are in high demand to meet regulatory requirements of recyclability. Several researchers confirmed that the continues natural fiber composites shows significant improvements in stiffness, strength and toughness of composites over fiber reinforced composites, which tends them to utilize in high-tech applications in various engineering sectors such as automotive, aerospace, defense etc. However, properties of the CNF composites depend on various factors matrix intrinsic properties, fiber properties, fiber volume fraction, and interfacial bond strength between the fiber and the matrix, fiber aspect ratio as well as fiber orientation (Ranganathan et al. 2015). In past decades, the CNF based composite have been developed through various manufacturing processes such as vacuum forming (Delvasto et al. 2010), extrusion (Grande and Torres 2005), injection moulding (Gao and Mäder 2006), and compression (Sreekumar et al. 2007). These conventional fabrication methods for composites require expensive facilities and equipment, such as autoclaves and complex rigid molds, hindering the wide applications. They were also constrained and limited by complexity structure, high mould cost, orientation, and alignment of fibers (Mohanty et al. 2004; Goodship et al. 2015; Faruk et al. 2012). In order to overcome the limitation, the novel method i.e. the 3D Printing/Additive technology for continuous fiber reinforced composites is being developed and studied by many researchers. 3D printing techniques are one of the most promising processes which enhance the light-weight and low-cost processing approaches for reinforced biocomposites when compared to several other composite fabrication methods (Compton and Lewis 2014).The research revolving additive manufacturing to develop structural components using continuous natural fiber composites is in its young stage and burgeoning exponentially throughout the globe. Several additive manufacturing techniques are being involved for the development of structural components which includes fused deposition modeling (FDM) or Fused Filament Fabrication (FFF), stereolithography (SLA), binder jetting (BJ), multi-jet fusion (MJF), selective laser sintering (SLS), selective laser melting (SLM), electron beam melting (EBM), electron beam additive manufacturing(EBAM), big area additive manufacturing (BAAM) (Dugbenoo et al. 2018); Out of which the FDM/FFF, SLA and SLS technologies are being used for developing polymeric composites. This chapter presents a detailed review of types of natural fibers and 3D printing techniques that are adopted by the researchers for the development of additive manufactured CNF-reinforced biocomposite parts. Additionally, based on the current technological significance and the limitation have been defined.

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2 Classification of Short, Long and Continuous Natural Fibers Biocomposites The natural fibers of cellulose/lignocelluloses fibers are derived from the plant, such as kenaf, hemp, jute, flax, ramie, nettle, pineapple leaf, sisal, date palm, cotton fire, coconut fibers, kapok, bamboo (Al-Oqla and Salit 2017; AL-Oqla and Sapuan 2018; TG YG et al. 2019). On the other hand, wool, silk, hair, and feathers are extracted from the largely consisted proteins of animals and some of the other waste of renewable resources (Saheb and Jog 1999). The natural fibers are classified as short, long and continuous fibers by its aspect ratio (fibers length and diameter) fibers, these fibers are reinforced with polymeric matrix and form biocomposites. Biocomposite materials are defined as composite materials in which at least one of the constituents is derived from natural resources. It is broadly categorized into three types. i.

Reinforcement of synthetic fiber such as carbon fiber, glass fiber, kevlar fiber into bio-matrix such as PLA, PHA, PHB etc. ii. Reinforcement of natural fibers such as jute, coir, sisal fiber etc. in synthetic/petroleum-derived polymers such as polyethylene (PE), polypropylene (PP) etc. iii. Reinforcement of bio/natural fibers such as jute, coir, sisal fiber etc. into biomatrix such as PLA, PHA, PHB etc. In particular, the natural fiber biocomposites fall under the category of (2) and (3), i.e. the biocomposite materials made from the combination of natural fibers reinforced petroleum-derived polymers or with bio-polymers. Globally, the cellulosic fibers are becoming very interesting for bio-based composite development as they possess advantages with their mechanical properties, low density, environmental benefits, renewability, and economic (Ngo 2018). The bio-based composites are majorly classified as short, Long and continuous fiber composites. In case of short fiber composites, the average size of fiber tends to range below 0.5 inches, whereas in long fiber composites the fibers are normally 0.5, 1.0, or 2.0 inches long (Collett and Campbell 2004) in both the cases namely the short fiber composites and long fiber composites, the fibers are discontinuous. But, in continuous fiber composites the fibers tend to be continuous owing its unique properties. Continuous-fiber composites normally have a preferred orientation, while discontinuous fibers generally have a random orientation. Therefore, the continuous fiber composites are used where higher strength and stiffness are required (but at a higher cost), and discontinuousfiber composites are used where cost is the main driver and strength and stiffness are less important (Chevali et al. 2010); The benefits and its properties of each category are described in Table 1.

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Table 1 The benefits and its properties of each category Description

Short Fiber Composites

Long Fiber Composites

Continuous Fiber Composites

Processing

Can be processed by many methods

Cannot be processed by many methods

Cannot be processed by many methods

Compression molding, Extrusion, Hand lay-up, Injection molding, Resin transfer molding, and Sheet molding, Additive Manufacturing

Compression molding, Direct long fiber extrusion technique (D-LFT), Hand lay-up, Resin transfer molding, and Sheet molding

Compression molding, Filament winding, Hand lay-up, Pultrusion, Resin transfer molding and Additive manufacturing

Length of fiber Lesser than 0.5 Inches

0.5–2.0 Inches

Larger than 2.0 Inches

Aspect ratio

Lower aspect ratio ( 100,000 Da), is the propagation reaction of cyclic monomers initiated by different ions. This process is carried out under mild conditions, with the presence of a catalyst. Three different stereoisomeric (L–lactide, D–lactide, and meso–lactide) are potentially produced during the formation of lactide from lactic acid. The ratio and sequences of D- and L-lactic acid monomers can eventually produce PLA with desired properties

thus producing more environmentally friendly and environmentally safe materials (Tezara et al. 2016). Among all bio-composites, natural fiber-reinforced PLA biocomposite has been extended to more challenging development to achieve the desirable properties and performances such as high-specific strength, biodegradability and excellent material rigidity (Siakeng et al. 2018). Table 5 summarizes natural fiber studies including their performance on mechanical, physical, and degradation properties.

5 Water Absorption Properties of Natural Fiber Reinforced PLA Bio-Composite Water absorption behaviour is one of the essential properties that must be considered in a bio-composite environment. Water absorption is a material’s ability to absorb water from its surroundings, and it plays a major role in determining the physical, chemical, and structural characteristics of the composite system (Celino et al. 2014).

Compression moulding

Compression moulding

PLA/cotton gin waste/flax fiber

PLA/jute bio-composite

Extrusion

Hemp Hurd/PLA bio-composite

Physical properties

Bio-composite laminate

PLA/coir fiber bio-composite

Mechanical properties

Manufacturing design

Bio-composite

Properties

Determine the water absorption behaviour of bio-composite and swelling in thickness after immersion in water.

Determine flexural properties of composite with different fiber loading

Determine tensile and flexural properties of composite and effect of the addition of compatibilizer agent, glycidyl methacrylate (GMA)

Determine the effect of fiber content (5-30 wt%) and fiber treatment on tensile and flexural properties

Purpose

(Bajracharyet al. 2017)

Reference

(continued)

PLA/jute bio-composite (Zandvliet et al. 2014) present far better water resistance compared to particleboard and hardboard. The swelling thickness of composite decrease as the natural fiber content decrease

Flexural modulus (Khan et al. 2015) increased 42% with the addition of 30 wt% cotton gin waste

The mechanical properties (Dong et al. 2014) of composite greatly improved in GMA grafted PLA/hemp hurd composite

20 wt% of treated coir fiber achieve optimum tensile and flexural properties

Result

Table 5 Mechanical, physical and degradation properties of Natural fiber reinforced PLA bio-composite

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Degradation properties

Properties

Table 5 (continued)

Mixing and compression moulding

Mixing and compression moulding, then composite is compositing into the soil

PLA/sisal fiber bio-composite

PLA/sisal fiber bio-composite

Vertical injection moulding

Extrusion

Starch/PLA/Agave Bagasse fiber bio-composite

PLA/sisal bio-composite

Manufacturing design

Bio-composite

Determine the thermal degradation of bio-composite

Determine biodegradation properties of bio-composite

Determine the water absorption properties of bio-composite

Determine water absorption behaviour based on thermoplastic starch (TPS)

Purpose

(Krishnaiah et al. 2018)

(Krishnaiahet al. 2018)

(Gracia et al. 2015)

Reference

Untreated PLA/sisal has (Rajesh, et al. 2015) high thermal degradation temperature compared to the treated PLA/sisal, and enzymatic environment increased rate of degradation composite

PLA/sisal degrade faster than the pure PLA and the combination of alkali and HIU treated fiber composites show slower degradation as compare to the untreated one

Water absorption is reduced significantly for combined treated PLA/sisal fiber bio-composites by 136% as compared to untreated composites

TPS hygroscopicity decrease when PLA and fiber content increase

Result

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Hence, the basic understanding of the water interaction with the macromolecule involved in the composite system is important for interpreting and predicting the fiber response when interacting in the matrix in a composite system. Although natural fiber provides low-cost production and environmentally sustainable material compared to synthetic fiber, the water-absorbing capacity of natural fiber-reinforced composites has become a major concern among researchers. In general, due to the hydrophilicity nature of plant fiber, water absorption of the natural fiber-reinforced composite is higher than synthetic fiber. Hydrophilic natural fiber can cause insufficient adhesion between matrix and fiber when reacting with hydrophobic matrix polymer in a composite system, leading to the poor performance of the composite (Melo et al. 2018). Moreover, it was established that the absorbed moisture would cause detrimental effects on the mechanical properties of the composite (Saikia 2010). Since most natural fiber is a hygroscopic and porous material, the interaction and distribution of water within the composite system plays a vital role in improving the long-term performance of the natural fiber-reinforced composite.

5.1 Sorption Mechanism Water absorbed by natural fiber reinforced composite exists in the form of free water and bound water. Free water is naturally adsorbed water, and not chemically bound to the fiber. It can pass through the void and crack spaces independently and can easily be removed by controlled heating or evaporation. Meanwhile, bound water disperses molecules of water which penetrate the cell walls and are bound to a polar group of materials (Petinakis et al. 2013). Once the natural fiber-reinforced composite is exposed to moisture, water molecules migrate through the composite and interact with the active fiber surface site consisting of the hydroxyl groups, thus forming intermolecular hydrogen bonds with fiber and thereby reducing interfacial adhesion between matrix and fiber. The deterioration cycle begins when the fiber starts to swell and develops tension in the interface regions resulting in its fragility and micro-cracking of the matrix around swollen fibers (Melo et al. 2018). The cracks exacerbate water absorption as it stresses the movement of moisture and the capillary by micro-cracks, contributing to fiber deterioration. Excessive absorption of water will increase bound water while diminishing free water. Therefore, the water-soluble material will gradually start to leach from the fiber and eventually cause ultimate debonding between the matrix and the fiber for prolonged exposure to moisture, biological activities such as fungal growth will encourage fiber degradation (Saikia 2010). Due to this process, researchers regarded water absorption as one of the limiting factors that reduce the composite’s efficiency. Some different fiber treatment has been implemented to minimize the damaging effects of moisture on the fiber-matrix interface (Girijappa et al. 2019). Figure 8 shows the moisture conditions in a composite’s fiber-matrix interface involving bound water and free water and the effects of moisture.

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Fig. 8 The effects of moisture in the fiber-matrix interface of the composite

5.2 Water Absorption Behaviour of Natural Fiber Reinforced PLA Bio-Composite Most of the prior research on water absorption behaviour of natural fiber reinforced PLA bio-composite deals with the hydrophilicity behaviour of plant fibers. Plant fiber is mainly composed of cellulose and hemicellulose that play a major role in moisture absorption. Since most of plant fibers are lignocellulosic fiber, moisture absorption may lead to fiber swollen, thus resulting in the dimensional stability changes (Celino et al. 2014). In some instances, moisture absorption also may affect the mechanical properties of bio-composite.

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Anuar et al. 2010 conducted a study on water absorption behaviour of biodegradable PLA-kenaf fiber bio-composite. In their study, PLA-kenaf samples were immersed in three different water which is seawater with a pH of 7.5, tap water with a pH of 7 and river water with a pH of 6. The amount of water absorbed by each sample is weighed every three days until 27 days to determine the rate of water absorption and physical properties of bio-composite. The results obtained from the study showed that the higher the fiber content and immersion time, the higher the amount of water absorbed by the composite. The author also further explained that this phenomenon is due to the basic nature of lignocellulosic fiber that consists of easy accessible hydroxyl group to form intermolecular hydrogen bonds, thus reduce interfacial adhesion between fiber and PLA matrix. Besides that, the authors also stated that the different types of water used in this study were not significantly affected the water absorption rate by PLA-kenaf bio-composite. Penjumras et al. 2015 have reported a similar trend in their study of water absorption behaviour of Durian Rind cellulose reinforced PLA bio-composite. In their study, water absorption behavior of two different loadings of Durian Rind cellulose (25 and 35 wt%) filled with PLA bio-composite was investigated by immersing the sample in water for 50 days. The results showed that the first 16 days of water immersion, a rapid increase in water absorption rate could be observed. After the next 16 days, the gradual increase of water uptake can be observed, before the composite finally reached an equilibrium point. The equilibrium point is the state where composite no longer absorbs water, and the weight of bio-composite remains unchanged. In order to overcome the drawback of high water absorption caused by lignocellulosic fiber, a variety of treatments and modifications are introduced to enhance the properties of fiber or composite. Nanthakumar et al. 2018 conducted a comparative study on water absorption properties of bleached and unbleached PLA/sugarcane leaves fiber (SLF) biofilm. The results showed that the water absorption of the unbleached biofilms increased from 5 wt% to 20 wt% of SLF loadings. However, the bleached biofilms showed the opposite result as an unbleached biofilm. It can be observed that the water absorption rate is lower in bleached biofilms. This is because bleaching treatment on SLF surface has improved the surface roughness of fiber, thus enhance filler-matrix adhesion by the interlocking mechanism. Generally, excellent filler-matrix adhesion can lessen the penetration of water molecules into biofilms, resulting in lower water absorption properties of biofilms. Besides that, Chung et al. 2018 also reported a study on water absorption resistance of an eco-friendly PLA/Kenaf bio-composite using acetylation. In their study, the surface of kenaf was modified using acetylation technique and then was compounded with PLA biopolymer using extrusion and injection molding technique. The hygroscopicity of composites was investigated by immersing the sample in the water at room temperature. The results showed that PLA filled with acetylated kenaf fibers showed lower water absorption compared to untreated PLA/kenaf bio-composite. Lower absorption of water must be due to the replacement of sorption sites consisting of hydroxyl groups with constituting kenaf with acetyl groups.

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Characterization of the Time of Phytosanitary Treatment of Frozen or Unfrozen Wood by Microwaves F. Erchiqui and H. Kaddami

Abstract The traffic and transportation of wood products around the world is one of the causes that have facilitated the migration of pathogens and insects between countries. These biological invasions have resulted in the destruction of plant species, causing significant economic and ecological losses in many parts of the world. To this end, the World Trade Organization, through the International Plant Protection Convention Working Group, has established a standard for phytosanitary measures (ISPM 15), which requires that wood products (pallets, packaging, etc.) be treated according to this standard. It is within this framework that this chapter is included and aims to present a numerical approach to quantify the time required for microwave phytosanitary treatment of wood products. For this purpose, the microwave heat conduction equation is expressed in terms of volumetric enthalpy and numerical resolution is achieved using the finite element method. As an application, we considered three Canadian wooden. For the analysis, we considered a frequency of 2466 MHz, temperatures from −20 to +20 °C and a moisture content of 131%. Keywords Phytosanitary treatment · Microwave · Orthotropic media · Thermal anisotropy · Enthalpy · Dielectric anisotropy · Finite element analysis

1 Introduction Circulations of some wood products are identified as one of the dissemination vectors and spread of non-native pathogens and insects in several countries of the world. This biological invasion has caused the destruction of plant species, causing significant economic and ecological disasters in many regions of the world. The Millennium F. Erchiqui (B) School of Engineering, Bioplasturgy Laboratory, University of Quebec in Abitibi-Temiscamingue, Université, Rouyn-Noranda, 445 boul, Québec, Canada e-mail: [email protected] H. Kaddami Department of Chemistry, Laboratory of Organomettalic and Macromolecular Chemitry, FST-Marrakech, Cadi Ayyad University, Avenue Abdelkrim Elkhattabi, B.P. 549, Marrakech, Morocco © Springer Nature Singapore Pte Ltd. 2021 M. T. Hameed Sultan et al. (eds.), Biocomposite Materials, Composites Science and Technology, https://doi.org/10.1007/978-981-33-4091-6_10

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Ecosystem Assessment classifies them as one of the main causes of biodiversity loss, along with other factors, such as habitat destruction, climate change and pollution. For example, the North American forest (its extern part) is still endangered by the non-native Asian long-horned beetle (Anoplophoraglabripennis) and the emerald ash borer (Agrilusplanipennis) (USDA 2003). According to the Food and Agriculture Organization (FAO), this invasion is due mainly the combined effects of the climate change and wide traffic of packaged products. In addition, the devastative risks of these species invasion, which involve many interactions between various spatial and temporal parameters, are difficult to quantify (Yemshanov et al. 2009). Recent estimates of the global economic impacts of these species exceed US$120 billion annually; this includes agricultural, forestry and public health impacts (USDA 2003). In Canada, the economic impact on the agriculture and forestry sectors has been estimated at Can $7.5 billion annually (Yemshanov et al. 2009). To face this new global reality, the International Plant Protection Convention (IPPC) working group promulgated the “International Standards for Phytosanitary Measures 15 (ISPM 15)” regulation in 2009 on the treatment of phytosanitary of all wood and wood-based materials. This standard aims to protect ozone layer and thus forbids polluting and fights against the international dissemination and spread of pathogens, fungi and insects that could endanger agricultures or ecosystems (FAO 2009; Fields et al. 2002). Thereby, ISPM-15 imposed and authorized thermal and chemical treatments of the circulating woods and packaging materials to destructs these pathogens (FAO 2009). Nevertheless, these treatments have economical and ecological side effects and drawbacks. For example Cooper et al. have pointed out the issue of recycling of these materials by raising out the health impact of reusing chemically sanitized wood and wood based materials (Cooper et al. 1996): What is the health hazard to burn borate or Imidacloprid sanitized firewood? What is the generated environmental impact and health hazard if chemically treated materials are recycled for application in outdoor conditions where are subjected to rainfall and sunlight (Nzokou et al. 2008a, b)? Dielectric heating treatment was ratified in ISPM 15 In 2013. It was proven that this heating technique is very effective in destroying pathogens (Nzokou et al. 2008a, b). Nevertheless, ISPM-15 has indorsed 56 °C as minimum temperature to destroy pathogens in piece of wood thicker than 6 mm when using this technique. In regard of the heating efficiency of this technique, one of the major advantages is the production of heat inside the material in a short period of time (Norimoto and Gril 1989), (Gašparík and Barcík 2013, 2014), (Oloyede and Groombridge 2000), (Hansson and Antti 2003). Nevertheless, the assessment of the heat distribution generated by dielectric heating in wood and the prediction of the time of its phytosanitary treatment is a very hard task. Indeed, these predictions are tribytary to many parameters such as (i) the non-isotropic dielectric, mechanical and thermal properties of wood, (ii) the product geometry and design, and (iii) its initial temperature and humidity, etc. Moreover, considering the diversity of wood based commercial products how is it possible to make sure of the total destroying of pathogens by dielectric heating treatment? At this stage, several uncertainties concerning the efficiency of such a treatment eliminating pathogens are to be checked. Thus, numerical

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simulation and modeling in predicting the processing time become challenging. In these conditions, a multi-physical model is needed for these simulation and modeling and its effectiveness is directly related to how mathematically Maxwell’s equations are coupled to conservation equations for wood? (Brodie 2007; Rattanadecho and Suwannapum 2009; Erchiqui et al. 2013; Erchiqui 2013a, b; Rattanadecho 2006; Zhu et al. 2007; Ni and Datta 2002). This raises a big challenge for numerical simulation since this has to consider the combined intricacies of mass and heat transfers, phase change as well as thermomechanical and electromagnetic interactions of a highly hygroscopic, thermally weak conductive and structurally anisotropy material as wood (Torgovnikov 1993; Norimoto and Yamada 1971). Several models are tested in literature to face these major challenges. The empirical model of Beer-Lambert is one of the most used models for Maxwell’s equations. This model is valid for semiinfinite medium and can quantify the microwaves energy loss in irradiated material (Erchiqui et al. 2013). Thus, an extensive question evolves: To which extend this model is still applicable for wood considered as finished anisotropic medium. In the case of microwaves as well as for radiofrequency, a criterion of applicability is provided in (Zhu et al. 2007; Erchiqui 2013a, b). This defines a critical thickness of wood sample to respect by considering the penetration depth (cm) of the radiation inside the sample. However, in the case of frozen wood, the ice melting (phase change) during microwave heating should be considered, especially in this case when the medium is considered as anisotropic. This evolves a difficult question from numerical modeling vantage point since one or more moving boundaries are to be considered. To overcome this issue, two ways can be used, in general: resolving independently the equations of energy for the liquid and solid phases, and considering the moving solid–liquid interface (Hu and Argyropoulos 1995a, b; Panrie et al. 1991), or resolving the enthalpy function considered as the equation of energy (Erchiqui et al. 2013). On the other hand, when a phase change is to be considered, several modeling approaches could be found in literature in the area of wood and food sciences (Ohlsson and Bengston 1971; Swami 1982; Rattanadecho 2006; Bhattacharya et al. 2002; Basak and Ayappa 1997; Coleman 1990). In the present work, which is a synthesis of work we carried out recently, we are treating the phytosanitary treatment of anisotropic frozen wood microwaves heating in order to numerically determine the time of application this kind of dielectric heating. To this end, a finite element 3D approach, involving the enthalpyvolume energy is elaborated. On the other hand, the wood sample is considered of parallelepiped geometry and the incident microwaves are plan and oriented perpendicularly to its faces. For such purpose, three Canadian wood species with different physico-chemical properties have been studied: trembling aspen (Populus tremuloidesMichx), white birch (Betula papyrifera) and sugar maple (Acer saccharum).

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2 General Considerations We assume that in this work the treatment by microwave of the wood species does not induce the degradation of its physical and mechanical properties. This aspect will be considered in this work since the highest temperatures that will be reached are far below the degradation temperature of wood. On the other hand, the focus will be on the numerical modeling aspect, through the consideration of nonlinear thermophysical and electrical properties. Also, the influence of the mass transfer will not be taken into account in this modeling work. The nonlinear heat conduction problem involving phase changes such as wood freezing is solved using three-dimensional volumetric specific enthalpy based on finite-element analysis. This study focused on a sample cube of wood 22 mm thick. We chose a moisture content of 131% for which the freezing phenomenon is pronounced. The choice of this high moisture content made the modeling process more delicate to handle with since one has to deal with water melting (phase change) during the heating process. The samples considered in this study derive from the external zone of the trunk (see Fig. 1). This choice is justified by geometrical considerations since for this part of the trunk one can easily make the assumption of cylindrical coordinates. On the other hand, this part of the wood (sapwood) is considered to have the tree’s storage function. This part of the wood, which is wanted by microorganisms and insects, is frequently subjected to pathogen attack, in contrast to the heartwood, which has a relatively natural resistance to attack by microorganisms owing to the extractives that are stored within its dead cells (disposal site for harmful by-products of cellular metabolism).

A tree trunk

Geometrical model for a tree trunk

Sample wood used in present modelling

Fig. 1 Wood sample’s cross section with illustrations the angle of the ring curves and of the radial and angular position

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3 Thermal Properties of Wood Three principal thermal conductive properties, namely thermal conductivity, specific heat and thermal diffusivity, are to be considered when modeling the heat process of wood. Generally, all three properties vary with the specific gravity SG, based on dry mass and green volume, the wood moisture content MC, expressed as a percentage/fraction of the ovendry mass of the wood and the temperature T. However, the ring orientation on a wood sample has a considerable effect on the rate of heat transfer. An example of a wood sample with curved rings is shown in Fig. 1. The two directions x and y shown in this figure are not exactly radial and tangential directions as defined by the wood anatomical structure; they will be called vertical and horizontal directions in the following. Accordingly, thermal conductivities in both directions on wood samples are usually different from the true radial and tangential direction. However, the thermal conductivities in the x and y directions can be calculated from the following formula: kx = −kr sin θ + kt cos θ and ky = kr cos θ + kt sin θ

(1)

Obviously, the thermal conductivity values change with the location, the θ angle and the radial position r of the sample. For illustrative purposes, one average angle, defined at the center of mass of the sample relative to the x-axis, is considered. For heat transfer analysis, we consider an angular position close to zero (see Fig. 1). Under these conditions, we have: kx ∼ = kt , ky ∼ = kr and kz = kl . The analytical expressions of the thermal conductivity, in the radial direction of the wood material, according to Kanter, are given in (Steinhagen and Harry 1988). The dependence of the thermal conductivity on temperature is generally low; it increases from 2 to 3% by 10 deg. For several species of wood, the ratio of longitudinal (kL) versus radial (kR) thermal conductivity is around 1.75 and 2.2, while the tangential thermal conductivity (kT) is usually slightly smaller (0.9 to 0.95 times) than the radial conductivity (Kanter 1975; USDA 1977). The specific heat capacity of wood, Cp, which represents the thermal energy required to produce one-unit change of temperature in one unit of mass, is sensitive to humidity. The presence of water improves the specific heat of the wood because the specific heat of the water is greater than that of the wood. For specific heat capacity, Cp, which represents the thermal energy required to produce one unit change of temperature in one unit of mass, an increase in moisture improves the specific heat of the wood because the specific heat of the water is greater than that of the wood. The analytical expressions, according to Steinhagen, are given in (Steinhagen and Harry 1988). Thermal diffusivity, which is a measure of how rapidly a material can absorb heat from its surroundings, is related to thermal conductivity, specific heat capacity and density (Kanter 1975; USDA 1977). Density (kg/m3 ), which represents the weight of wood divided by the volume at a given moisture content, is one of the most important physical properties. The analytical expression for density considered in this work is that given in (Simpson and Tenwolde 1999).

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Finally, to study wood heating (thawing or not), it is important to quantify the value of the latent heat of wood species. For this purpose, we can use the formula provided by Chudinov (1965):  L = LW

MC − 30% MC + 100%

 (2)

Lw is the latent heat of water fusion (334 kJ/kg).

4 Complex Dielectric Properties of Wood Propagation of electromagnetic waves in materials is determined by their electrical and dielectric parameters. In the case of dielectrics, the √ parameter of greatest interest is the complex permittivity, ε (= ε˙ − jε ), where j = −1. This parameter describes the ability of the material to support an electric field: the real part of ε, known as the dielectric constant, describes the ability of a material to store energy and the imaginary part of the ε, known as the loss factor, describes the ability of a material to dissipate energy, which naturally results in heat generation. Usually, depending on the type of material, the permittivity varies with each phase of the material, since the concentration of molecules and their bonding in the material differ. Several theories have been proposed to describe the permittivity of dielectrics from the constituent elements, concentration and particle size and shape (Zielonka and Gierlik 1999; Phillips et al. 2001). For wood material, complex permittivity varies with the type of wood species, density, frequency, moisture content, temperature and structural orientation. From the electromagnetic point of view, it is desirable to refer to the complex relative permittivity, εr , as being its permittivity with respect to that of free space, ε0 , such that ε = εr ε0 . For wood material, according to (Peyskens et al. 1984), the values of the longitudinal dielectric constant (εL ) are 1.25–3 times higher than the tangential dielectric constant ones (εT ), while the ratio of the tangential versus radial dielectric constant (εR ) is about 0.9 and 1.25. With regard to the interactions between the material and the electric field, two physical parameters are of primary interest: the absorption and storage of electric potential energy within the dielectric material, and the dissipation or loss of part of this energy when the electric field is removed (James 1975). This dissipation of energy in the material induces effects on heat and mass transfer. To this end, note that the conductive heat process by microwave energy in different directions of wood has two important characteristics: (i) the amplitude of the oscillation of electromagnetic power distribution and (ii) the magnitude of the heat transfer rate. A rigorous mathematical formulation of the heating process requires a good knowledge of the power flux (Poynting vector) associated with electromagnetic microwave propagation, which is a solution of Maxwell’s equation in anisotropic dielectric material.

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For anisotropic dielectric wood, both the electric charge density (ρ) and the electric currents (J) are equal to zero. Furthermore, the following linear interrelations can be established between the electric and magnetic properties (Pozar 2011): D = ε · E and B = μ · H

(3)

where E, B, H, and D are the electric field (V m−1 ), the magnetic field (Wb m−2 ), the magnetic induction (A m−1 ), and the electric displacement (C m−2 ), respectively. ε and μ are, respectively, the relative dielectric and permeability tensors. For wood, it is generally believed that the magnetic permeability tensor μ is closely approximated by the real tensor μ0 I (I is the identity tensor) (Erchiqui 2013a, b). μ0 (H/m) is the permeability of vacuum. Rotation of the electric field vector E on 180° does not change the dielectric properties of wood materials (Pozar 2011). So, when E is arbitrarily oriented in space and forms an angle θ1 with the longitudinal direction, angle θ2 with the transversal direction, and angle θ3 with the tangential direction, closed-form expressions for calculating the relative dielectric constant, ε , and the dielectric loss tangent, tan δ, is given in (Peyskens et al. 1984).

5 Enthalpy Model for Heating of Orthotropic Media In anisotropic materials, thermal energy flows at different rates along different directions. This is taken into account by assigning a second-order tensor character to the thermal conductivity. The x-component of the heat flux vector qx becomes (Bergman et al. 2011):   ∂T ∂T ∂T + kxy + kxz qx = − kxx ∂x ∂y ∂z

(4)

and similarly for the other two directions. For orthotropic materials, heat still travels at different rates along different directions but the heat flux along any direction is driven only by the temperature gradient along that direction, i.e. kij = 0 when i = j. The heat equation then becomes ρCp

      ∂ ∂T ∂T ∂T ∂T ∂ ∂ = kxx (T ) + kyy (T ) + kzz (T ) ∂t ∂x ∂x ∂y ∂y ∂z ∂z

(5)

In this paper, we solve the energy equation in terms of the enthalpy function and assume that the interface  between the solid (− ) and liquid (+ ) phases can be described by a regular function F (x, y, z, t) = 0 (Hu and Argyropoulos 1995a, b). The temperature in the solid and liquid phases is denoted by T s and Tl respectively. Using a temperature-based formulation, the two energy equations that govern both solid and liquid media are written in the absence of convective motion:

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F. Erchiqui and H. Kaddami

∂Ts ρ s (Ts )Cps (Ts ) ∂t       ∂ ∂ ∂ s ∂Ts s ∂Ts s ∂Ts = k + k + k , F(x, y, z, t) < 0 ∂x xx ∂x ∂y yy ∂y ∂z zz ∂z ∂Tl ρ l (Tl )Cpl (Tl )  ∂t      ∂ ∂ ∂ ∂Tl l ∂Tl l ∂Tl = kxx + kyy + kzzl , F(x, y, z, t) > 0 ∂x ∂x ∂y ∂y ∂z ∂z

(6)

(7)

where ρs [kg/m3 ], Cps [J/kg/°C] and ks [W/m/°C] are respectively the density, the specific heat capacity and the thermal conductivity of the material in the solid state. ρl [kg/m3 ], Cpl [J/kg/°C] and kl [W/m/°C] are respectively the density, specific heat capacity and thermal conductivity of the material in the liquid state. The boundary conditions, also known as the Stefan conditions (Hu and Argyropoulos 1995a, b), allow us to consider the energy jump between the two phases: Ts (x, y, z, t) = Tl (x, y, z, t); F(x, y, z, t) = 0

(8)

    ∂Ts s ∂Ts s ∂Ts ∂Tl l ∂Tl l ∂Tl · ∇F − kxl (Tl ) · ∇F kxs (Ts ) , ky (Ts ) , kz (Ts ) , ky (Tl ) , kz (Tl ) ∂x ∂y ∂z ∂x ∂y ∂z ∂F = −ρ.L (9) ∂t

L is the latent heat [J/kg]. In order to avoid numerical problems in the phase transition region, the variables were changed. The temperature-dependent density ρ(T) and specific heat capacity Cp (T) can be replaced by the volumetric specific enthalpy H(T) defined by (Nedjar 2002): T H (T ) =

ρ(T )cp (T )dT

(10)

Tref

where Tref [°C] is a reference temperature. The advantage of using an enthalpy—rather than a temperature-based formulation is that it simultaneously eliminates the doubling of the energy equation and the Stefan conditions. By introducing the volumetric specific enthalpy H(T) at the phase transition, the abrupt jump in the volumetric heat capacity is transformed into a relatively smoother temperature-dependent function. It can be shown that, around phase transition, the curvature of volumetric specific enthalpy H(T) is similar to that of thermal conductivity k(T). H(T) depends strongly on the type of material, although a clear distinction must be made between the following two situations. In the first f case, the phase change occurs in an interval [Ts1 −T2 ]. In this case the enthalpy is

Characterization of the Time of Phytosanitary Treatment …

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defined by:   H T ≤ T1s =

T ρ s (T )Cps (T )dT

(11)

Tref T T  1  ∂L f s s s H T1 < T ≤ T2 = ρ (T )Cp (T )dT + ρ(T ) dT , ∂T s

(12)

T1s

Tref

T T  1  f s s H T > T2 = ρ (T )Cp (T )dT + ρ(T )L + ρ f (T ).Cpf (T )dT s

Tref

(13)

f T2

In the second case, the phase change occurs at a constant  temperature  so the f enthalpy exhibits discontinuity at the fusion temperature, Tm = Ts1 = T2 , and is defined as follows: T H (T ≤ Tm ) =

ρ s (T )Cps (T )dT

(14)

Tref

Tm

T ρs (T)Csp (T)dT + ρ(T)L +

H (T > Tm ) = Tref

ρf (T).Cfp (T)dT

(15)

Tm

Zero enthalpy is defined at the saturated solid temperature. For purposes of numerical simulation, the enthalpy method is more reliable. However, the problem remains difficult and highly nonlinear. Following a procedure similar to the enthalpy transform method, the temperature-dependent thermal conductivity can be substituted by the thermal conductivity integral, using Kirchhoff’s transformation: T θx (T ≤ Tm ) =

s kxx (T )dT ; Tref

T θy (T ≤ Tm ) =

s kyy (T )dT ; Tref

T θz (T ≤ Tm ) =

kzzs (T )dT ; ∀T Tref

(16)

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F. Erchiqui and H. Kaddami

T θx (T > Tm ) =

l kxx (T )dT ; Tm

T θy (T > Tm ) =

l kyy (T )dT ; Tm

T θz (T > Tm ) =

kzzl (T )dT

(17)

Tm

To do so, we consider Leibniz’s rule (Flanders 1973) for differentiation under the integral sign in Eqs. (14)–(15) and (16)–(17): ∂T ∂H = ρ(T )cp (T ) ∂t ∂t ∂θx ∂T = kxx (T ) ; ∂x ∂x

(18)

∂θy ∂T = kyy (T ) ; ∂y ∂y

∂θz ∂T = kzz (T ) ∂z ∂z

(19)

With these transformations, and taking into account the internal volumetric heat generation of microwave energy Pwave , the governing heat equation reduces to a partial system, with two mutually related dependent variables H and   differential θ = θx , θy , θz : ∂H (T ) = ∂T



∂ 2 θy ∂ 2 θz ∂ 2 θx + + ∂x2 ∂y2 ∂z 2

 + Pwave

(20)

For microwaves sources, the internal volumetric heat generation of microwave energy Pwave is given by the following expression Pwave (T ) = −Re(∇ · S)

(21)

where S is the instantaneous Poynting vector. To solve the problem, we introduce the boundary condition into Eq. (22) as follows:   ∂θy ∂θx ∂θz + ny + nz + h(T − T∞ ) − q · n = 0 nx ∂x ∂y ∂z

(22)

where q [W/m2 ] is the radiative heat flux incident, n is the outward normal (nx , ny , nz ) to the surface h, [W/m2 /°C] is the surface heat transfer coefficient, and T∝ is the temperature of the surrounding medium (air). The term h (T–T∝ ) represents the convection heat transfer from the material to the environment. The incident heat flux depends on the source configuration and the position of the material. The advantage

Characterization of the Time of Phytosanitary Treatment …

283

of using an enthalpy- rather than a temperature-based formulation is that it simultaneously eliminates the doubling of the energy equation and the Stefan conditions (Erchiqui 2013a, b).

5.1 Implicit Time Integration Scheme Numerical time approximation schemes are used mainly to obtain the transient response. These numerical integration schemes derive recursion relations that relate H(t) at a moment of time t to H(t + t) at another moment of time t+ t. The solution is then solved step by step starting from the initial conditions at time t = 0 until the desired duration of the transient response is calculated. The most common numerical schemes for the solution of Eq. (20) belong to the weighted Euler difference family of time approximations, as follows (Dokainish and Subbraj 1989): H n+θ = (1 − θ )H n + θ H n+1 , with : θ =

t − tn t

(23)

The parameter θ varies in the range [0–1]. The θ schemes are unconditionally   stable when θ ≤ 1/2 and O( t) are accurate, with the exception of the O t2 – convergent Crank–Nicolson scheme (θ = 1/2). Setting θ = 1 leads to the backward Euler (fully implicit) scheme, which is only first-order accurate but very stable and hence ideally suited for integration. In the present study, we consistently use the semi-implicit Crank–Nicolson scheme (Dokainish and Subbraj 1989). In this case, Eq. (24) becomes:   ∗ ∗ H n+1 = K ∗n .H n + G∗n .H 2n + R∗n,n+1 K n+1 + Gn+1

(24)

where K* , G* and R* are modified global matrices and Hn+1 is the vector of global nodal enthalpies at moment tn+1 .

6 Electromagnetic-Wave Energy and Poynting’s Theorem The time-dependent power flow density of an electromagnetic wave is given by the instantaneous Poynting vector S (Pozar 2011): S=

1 E × H∗ 2

(25)

E and H* are the electric field (V m−1 ) and the conjugate magnetic field intensity (A m−1 ), respectively. In general, we are dealing with steady-state harmonic timevarying fields. It is convenient to represent each field vector as a complex phasor by

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applying a Fourier transform. Assuming that we have a monochromatic wave, we can write: E(r, t) = e−jωt E(r)

(26)

where E is a complex vector and a function of r (m), and ω (radian s−1 ) is the angular frequency of the incident radiation. The other field vectors can be written using the same notation as in Eq. (26).

6.1 Maxwell’s Equations and Power Dissipation A propagating electromagnetic wave is composed of oscillating electric (E) and magnetic (H) fields. The space and time dependence of these fields are described by the following Maxwell’s equations: ∇ ×E =−

∂B , ∂t

∇ ×H =J−

∂D , ∂t

(27) (28)

where E, B, H, J and D are the electric field (V m−1 ), the magnetic field (Wb m−2 ), the magnetic induction (A m−1 ), the current density (A m−2 ), and the electric displacement (C m−2 ), respectively. Considering the definition of the Poynting vector S, as in (25), and using both the identity vector and Maxwell’s equations with the complex conjugate field H, (29) can be derived from (27) and (28) as follows: ∇ ·S=

  

1  1 ∇ · E × H∗ = − E · ∇ × H∗ − H∗ · (∇ × E) 2 2  ∂D∗ ∂B 1 + E · J∗ + H∗ · =− E· 2 ∂t ∂t

(29)

The vector S depends on the explicit electric currents, the temporal variations of the electric displacement, and the magnetic induction.

6.2 The Case of Anisotropic Dielectric Materials For anisotropic dielectric wood, both the electric charge density (ρ) and the electric currents (J) are equal to zero. Furthermore, the following linear interrelations can be established between the electric and magnetic properties:

Characterization of the Time of Phytosanitary Treatment …

285

D = ε0 ε · E

(30)

B = μ0 μ · H

(31)

J=0

(32)

where ε0 is the dielectric permittivity (= 8.8541 × 10−12 F/m) of the free space and μ0 is the permeability of vacuum (= 4π × 10−7 ∼ = H/m). ε and μ are, respectively, the relative dielectric and permeability tensors: ε = Re(ε) − jIm(ε) and μ = Re(μ) − jIm(μ)

(33)

with: ⎡

⎡    ⎤ ⎤    εLR εLT εLL εLL εLR εLT    ⎦, Im(ε) = ⎣    ⎦ Re(ε) = ⎣ εRL εRR εRT εRL εRR εRT       εTL εTR εTT εTL εTR εTT

(34)

and ⎡

⎡  ⎤ ⎤ μLL μLR μLT μLL μLR μLT Re(μ) = ⎣ μRL μRR μRT ⎦, Im(μ) = ⎣ μRL μRR μRT ⎦ μTL μTR μTT μTL μTR μTT

(35)

The subscripts L, R, and T are respectively the longitudinal, radial, and tangential directions. In general, we are dealing here with steady-state harmonic time-varying fields. It is convenient to represent each field vector as a complex phasor by applying a Fourier transform. Assuming that we have a monochromatic wave, we can write: E(r, t) = e−jωt E(r),

(36)

where E is a complex vector and function of r, and ω is the angular frequency of the wave. The other field vectors can be written using the same notation of Eq. (36). Following the application of (30)–(32) and (36), (29) will be reduced to the equation below: ∇ · S = −j

    ω   ∗ ∗ ε0 E · ε · E + μ0 H · μ · H 2

(37)

Introducing the complex expressions of the dielectric permittivity and the magnetic permeability tensors, we get from (37): Re(∇ · S) = −

   ω  ∗ ∗ ε0 E · Im(ε) · E + μ0 H · Im(μ) · H 2

(38)

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F. Erchiqui and H. Kaddami



 1   1  ∗ ∗ Im(∇ · S) = −2ω ε0 E · Re(ε) · E + μ0 H · Re(μ) · H 4 4

(39)

Note that (38) shows that the real part of (∇.S) is equal to the power dissipated per unit volume in the form of heat. The imaginary part of the complex (∇.S), Eq. (39), is equal to 2ω times the net reactive power stored per unit volume and time-average of electric We and magnetic Wm . energy densities (Hollis 1983): ⎛ 1 We = Im⎝ T

⎞ We dt ⎠ =

0

⎛ Wm = Im⎝

T

1 T

T

⎞ Wm dt ⎠ =

 1  ∗ ε0 E · Re(ε) · E 4

(40)

 1  ∗ μ0 H · Re(μ) · H 4

(41)

0

where We and Wm are respectively the instantaneous electric and magnetic energy densities, and T is the period. The quantity ( We + Wm ) (J/m3 ) is the total time average stored energy density of the electromagnetic field in the anisotropic dielectric material. The real part of the complex (∇.S), is equal to 2ω times the net power dissipated per unit volume and time-average of electric We and magnetic Wm energy densities: Pwave =

   ω  ∗ ∗ ε0 E · Im(ε) · E + μ0 H · Im(μ) · H 2

(42)

ε0 is the dielectric permittivity (= 8.8541 × 10−12 F/m) of the free space and μ0 is the permeability of vacuum (= 4π × 10−7 ∼ = H/m). ε and μ are, respectively, the complex relative dielectric and complex permeability tensors. In wood systems, the magnetic permeability tensor μ is closely approximated by the real tensor μ0 I ; (I is the identity tensor), (Kaestner and Bååth 2005). This is what we assumed in this paper. Thus, the power dissipated per unit volume, as in (42), becomes: Pwave =

 ω  ε0 E · Im(ε) · E 2

(43)

Computing the power dissipation involves determining the electric field as a function of the position within the material. If E is parallel with one of the three principal directions (longitudinal, radial or tangential) of the wood sample, the power dissipated per unit volume may be simplified as (see Appendix): Pnet =

ω  2 ε0 εd Ed 2

(44)

where εd is the dielectric constant in the principal direction d: longitudinal (L), radial (R) and tangential (T). Assuming the electro-neutrality of wood (∇(∇ · E) = 0), we

Characterization of the Time of Phytosanitary Treatment …

287

deduce for each principal direction from Maxwell’s equations the expression of Helmholtz’s equation of wave propagation (Erchiqui 2013a, b): ∇ 2 E − γ2 E = 0

(45)

where γ is the constant complex propagation γ = α+jβ, β is the attenuation constant and α is the phase (a constant). These parameters are related to the dielectric properties of the material and frequency of radiation by: ω α= c

  ε  2

1+

tan2

   ω ε  δ+1 β = 1 + tan2 δ − 1 c 2

(46)

√ c = 1/ μ0 ε0 is the speed of light. The term δ (= tg−1 (ε /ε )) is the dielectric loss angle. The attenuation constant, β, controls the rate at which the incident field intensity decays into a sample. 1/(2β) is known as the penetration depth (d). The phase constant, α, represents the change of phase of the propagation radiation and is related to the wavelength of radiation by λ = 2π/α.

6.3 Uniform Plane Wave Propagation and Power Dissipation   In this paper, we assume that each component of the electric field E = Ex , Ey, , Ez is a uniform plane microwave. Also, each component of a wave is assumed to be incident normally on opposite faces of the sample: x direction: longitudinal direction L Ex = Ex (z)ax

(47)

Ey = Ey (x)ay

(48)

y direction: radial direction R

z direction: tangentiel direction T Ez = Ez (y)az

(49)

where x is a component of the electric field (Ex ), which is a function of the parameter z, y is a component of the electric field (Ey ), which is a function of the parameter x, and z is a component of the electric field (Ez ), which is a function of the parameter y (see Fig. 2). For each direction, according to Eq. (45), we have the following expression for Helmholtz’s equation of wave propagation:

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F. Erchiqui and H. Kaddami

Fig. 2 Schematic of sample wood exposed to plane microwaves from the three principal faces

Longitudinal direction: d 2 Ez az − γy2 Ez az = 0, 0 ≤ y ≤ Ly dy2

(50)

d 2 Ey ay − γx2 Ey ay = 0, 0 ≤ x ≤ Lx dx2

(51)

Radial direction:

Tangential direction: d 2 Ex ax − γz2 Ex ax = 0, 0 ≤ z ≤ Lz dz 2

(52)

Lx , Ly and Lz are, respectively, the length of the sample wood in the x, y and z directions (see Fig. 1). The unit vectors az , ay and ax are, respectively, the normal to the surface (AEFB), (ADHE) and (HCTE) of the sample wood (see Fig. 2). The waves travel through the material from right to left with an incident power flux I0 . The regions external to the sample are denoted by index 1 for the left and index 3 for the right. The material is denoted by index 2. The exact solutions for the power absorbed of each principal direction of material are given respectively by the following expressions (Erchiqui 2013a, b): Longitudinal direction: ⎡

x Pwave

 2 ⎤  x x x x x x e−2β x + R23 e−4β Lx e2β x + 2R23 e−2β Lx cos 2α x (x − Lx )−x23 ⎢ ⎥ = 2I0x β x ⎣  2  2  −2β x L ⎦ (53) x x x x x x xL −4β x x x 1 + R12 R23 e − 2R12 R23 cos 12 +23 +2α Lx e

Characterization of the Time of Phytosanitary Treatment …

289

Radial direction: ⎡

y

Pwave

 2    y ⎤ x y y y y + R23 e−4β Ly e2β y + 2R23 e−2β Ly cos 2α y y − Ly −23 ⎥ y ⎢ = 2I0 β y ⎣  2  2  −2β y L ⎦ (54) y y y y y y yL −4β y y y 1 + R12 R23 e − 2R12 R23 cos 12 +23 +2α Ly e e−2β

yy

Tangential direction: ⎡ z Pwave

⎢ = 2I0z β z ⎣

 2  ⎤ z z z z z z e−2β z + R23 e−4β Lz e2β z + 2R23 e−2β Lz cos 2α z (z − Lz )−z23 ⎥ ⎦ (55)  2  2   z z z z z z 1 + R12 R23 e−4β Lz − 2R12 R23 cos z12 +z23 +2α z Lz e−2β Lz

The superscripts x, y z are associated respectively with the longitudinal, radial and tangential directions. θij is the phase angle for reflection coefficient at the interface between layers i and j. Rij is the absolute value of the complex reflection coefficient.

7 Validation of the Proposed Method To validate the proposed approach, three situations are considered: (i) heat transfer in one-dimensional raw beef sample exposed to microwave energy (Ayappa et al. 1991), (ii) heat transfer in an orthotropic plate with imposed temperatures (Bruch and Zyroloski 1974) and (iii) heat transfer of frozen wood with phase change: imposed temperatures (Steinhagen and Harry 1988).

7.1 Numerical Heating Validation To validate the specific volumetric enthalpy-based finite element developed in this work, we consider the results obtained in for a raw beef sample exposed to microwave radiation. In this example, the author solves an isothermal heat conduction equation where the dielectric and thermophysical properties are constants. For our study, we consider a linear mesh with 42 elements and 41 nodes. Figure 3 compares the power distribution and the predicted temperature profile for the raw beef sample exposed to radiation from the left face. The results obtained in (Ayappa et al. 1991) are similar to ours for both formulations.

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Fig. 3 Influence of slab thickness on power and predicted temperature for raw beef sample exposed to microwave from the left, I0 = 3 W/cm2 , f = 2450 MHz

7.2 Analytical Heat Transfer in an Orthotropic Plate: Imposed Temperatures For analytical validation, we consider an orthotropic plate subjected to a uniform temperature. The temperature T (x, y, t) across the thickness with time t is computed

Characterization of the Time of Phytosanitary Treatment … Table 1 Geometrical and thermophysical properties of material

Density ρ (kg/m3 ) Specific heat Cp (J

291 1.0

kg−1

°C−1 )

1.0

Thermal conductivity kx , x direction, (W m−1 °C−1 )

0.2

Thermal conductivity ky , y direction, (W m−1 °C−1 )

0.4

Length in x direction (m)

1.0

Length in y direction (m)

1.0

using Laplace transforms to solve the 2D energy equation with an imposed temperature (Bruch and Zyroloski 1974). The thickness as well as the physical properties of material used in the numerical validation is given in Table 1. For the finite-element analysis, we consider a quadrilateral mesh with 400 elements and 441 nodes. In the case where the initial temperature of the plate is 30 °C and the imposed temperature is 0 °C, the analytical solution is given by the following equation (Bruch and Zyroloski 1974): T (x, y, t) =

∝  ∝  n=1 j=1

    ky j 2 π 2 kx n2 π 2 t nπ x jπ y An sin sin exp − + (56) Lx Ly L2x L2y ρCp

where: 





120 n j An = (−1) − 1 (−1) − 1 π 2 jn

(57)

where kx and ky are respectively the thermal conductivity in the x and y directions. Lx and Ly are respectively the length and width of the plate. Table 2 compares, at t = 0.25 s, the predicted temperature obtained by the theoretical equation with the volumetric enthalpy based on the finite-element method. The results illustrate the excellent agreement between the theoretical and numerical solutions. The agreement is very good with an error of less than 1.0%. For greater precision, a more refined mesh is necessary.

7.3 Experimental Heating Validation: Transient Heating of Frozen Logs In this section, firstly, we consider the experimental temperature measurements obtained for transient heating of frozen logs (Steinhagen and Harry 1988), then, those obtained in (Peralta and Bangi 2006). For both tests, it involves heating a tree trunk immersed in water at a temperature of 54 °C. In the first case, it is trembling aspen log (radius is 0.3175 m and its initial temperature is −22 °C). In the second

292

F. Erchiqui and H. Kaddami

Table 2 Temperature distribution at time = 0.25 s X (m) Y (m)

0.0

0.2

0.4

0.6

0.8

1.0

0.0

0.000a

0.000a

0.000a

0.000a

0.000a

0.000a

0.000

0.000

0.000

0.000

0.000

0.000

0.2

0.000a

3.832a

6.196a

6.196a

3.832a

0.000a

0.000

3.864

6.250

6.250

3.864

0.000

0.4

0.000a

6.121a

9.898a

9.898a

6.121a

0.000a

0.000

6.163

9.969

9.969

6.163

0.000

0.6

0.000a

6.121a

9.898a

9.898a

6.121a

0.000a

0.000

6.163

9.969

9.969

6.163

0.000

0.8

0.000a

3.832a

6.196a

6.196a

3.832a

0.000a

0.000

3.864

6.250

6.250

3.864

0.000

1.0

0.000a

0.000a

0.000a

0.000a

0.000a

0.000a

0.000

0.000

0.000

0.000

0.000

0.000

a Numerical

results with ThermoForm code

test, it is eastern white pine log (the trunk radius is 0.2285 m and its initial temperature is −23 °C). In (Steinhagen and Harry 1988), it is assumed that logs is subjected to radial heating. In (Peralta and Bangi 2006), it is assumed that logs is subjected to orthotropic heating (radial and longitudinal). For both tests, the heating time is 60 h. The thermo-physical properties of the wood material are given in (Steinhagen and Harry 1988) and (Peralta and Bangi 2006) respectively. For finite element analysis, we consider a quadrilateral mesh (see Fig. 4). In Fig. 5, we presented the history of the temperature obtained numerically and experimentally in the center of the trembling aspen trunk (Steinhagen and Harry Fig. 4 Circular (2D) geometry mesh

Characterization of the Time of Phytosanitary Treatment …

293

Fig. 5 Experimental end numerical temperature versus heating time at the centre of log

1988). Figure 6 illustrates views of the numerical temperature distribution at different times: 900, 1800, 2700, and 3600 min. In Fig. 7, we presented the temperature against heating time for three different points located 22.9 cm (location 1), 10.2 cm (location 2), and 2.5 cm (location 3) from the surface of an eastern white pine log (Peralta and Bangi 2006). The numerical results for the heating of frozen wood showed an excellent agreement with the experimental data, in both cases of tests.

8 Numerical Application: Characterization of the Time of Phytosanitary Treatment Wood by Microwaves This study focused on 22-mm-thick boards. This dimension corresponds to the standard value for boards used in the manufacture of EUR EPAL® pallets. For this, a 3D wood log is considered for three wood species: trembling aspen, white birch and sugar maple. The log is considered to be orthotropic and its thermal and dielectric properties are functions of temperature, moisture content and structural orientation. The log structure is square (Lx = Ly = Lz = 2.2 cm). The principal orientations in sample woods are indicated by x for the longitudinal direction (L), y for the radial direction (R), and z for the tangential direction (T). To analyze the thermal and dielectric anisotropy effect on thawing frozen wood using microwave energy, we considered the following situation: kL = 1.8kR and kT = 0.9kR

(58)

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F. Erchiqui and H. Kaddami

Fig. 6 Temperature versus heating time at the centre of log

εL = 1.8εR and εT = 0.9εR

(59)

The wood material is exposed to radiation of equal intensity (1 W/cm2 ) and frequency (2466 MHz from six faces): two faces in the x direction, two faces in the y direction, and two faces in the z direction. The initial wood temperature varies from −20 to 20 °C. The no-isothermal complex dielectric properties of the three wood species, for a moisture content (MC) at 131% (MC), are given in (Erchiqui et al. 2020). The specific gravity (SG) is 0.32, 0.48 and 0.55 for aspen, white birch and sugar maple respectively. The no-isothermal longitudinal relative dielectric constant ε and the relative dielectric loss ε are given in reference (Erchiqui et al. 2020). In this application, the density (ρ), specific heat capacity (Cp ) and radial conductivity (kR ) are calculated by the formulas provided in (Steinhagen and Harry 1988). Figure 8 presents, at time 200 s and initial temperature −20 °C, different views of the surface temperature distribution induced by microwave treatment of the aspen trembling, birch white and sugar maple. At this time, Fig. 9 presents, for the trembling aspen log on the half-planes of symmetry (longitudinal, radial and tangential directions), the profiles of temperature distribution predicted from the power dissipation computed from Maxwell’s equations.

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Fig. 7 Experimental and numerical temperature versus heating time for three different points located from the surface of an eastern white pine log

Figure 9(b–d) show different views of the temperature distribution in the longitudinal, radial and tangential directions, respectively. Similarly, Figs. 10 and 11 present the temperature distribution profiles for sugar maple and white birch, respectively. Significant differences can clearly be seen in temperature distribution when comparing the three directions and the three studied woods. In fact, for aspen and sugar maple, the longitudinal direction of the wood is preferred to temperature increase. On the other hand, for white birch, we note that the temperature profile associated with the longitudinal direction dominates at the edges and the center of the sample. Beyond these regions, for white birch, the temperature profile associated with the tangential direction dominates. One can also note that the highest temperatures were reached for the aspen wood sample, followed by white birch. From the thermo-physical point of view, since the thermal conductivity and specific heat of each of the three hardwoods involve the same formulas, see reference (Steinhagen and Harry 1988), these differences could only be attributed to the differences in the density of these woods. As illustrated in Table 3, aspen is lighter while sugar is the densest wood. Indeed, the heating time increases with the density. This confirmation is due to the dependence of the volume enthalpy on density. In the melting zone (solid ice), the latent melting enthalpy of fusion is proportional to the density (ρL ). Also, from the electromagnetic point of view, the oscillatory nature of the electromagnetic wave is another parameter which affects the temperature profile. Indeed, the propagation of electromagnetic radiation in a wood medium is a result of

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Birch white

Aspen trembling

Sugar maple Fig. 8 Surface temperature distribution for aspen trembling, white birch and sugar maple, exposed to microwave at time = 200 s, I0 = 1 W/cm2 , f = 2466 MHz, T0 = −20 °C

a transmitted microwave at the incident face and a reflected microwave from the end surface. This situation, which affects the quality of the signal of the power absorbed by the wood, also affects the temperature profile. Indeed, a slight difference between the dielectric properties in the radial and tangential directions can have significant effects on the temperature profiles and power distributions. Consequently, it is very difficult to predict, from the complex dielectric and thermophysical properties of wood, the evolution of the temperature profile. Under these conditions, the experimental tools for characterization and the numerical modeling (mass transfer, phase change and thermomechanical and electromagnetic interactions) must be considered. For the following analysis, we denote TA, WB and SM the trembling aspen, white birch and sugar maple. Figures 12, 13 and 14 represent the times required to eliminate 10, 25, 50, 75, and 100% of pathogens by microwave energy from trembling aspen,

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Fig. 9 a Temperature profiles distribution and views of the temperature distribution at half-planes of symmetry of b longitudinal, c radial and d tangential direction for aspen trembling sample exposed to microwave at time = 200 s, I0 = 1 W/cm2 , f = 2466 MHz, T0 = −20 °C

white birch, and sugar maple, respectively for initial temperatures of −15, +0 and 15 °C. The percentage of disinfected sapwood is defined as the fraction of wood that reaches the temperature of 56 °C. One can easily note that the required treatment time to reach 100% disinfection increases when the initial temperature decreases. On the other hand, the treatment time of complete disinfection by microwave energy is significantly higher when the initial temperature is below zero. This finding is explained by the accumulation of latent energy in the heating of the solid ice present in the material. Once exceeded, the material temperature increases more easily (thermal conductivity in the liquid region is high). Figure 15 summarizes various minimum times required to achieve 100% disinfection as a function of initial temperature and wood type for lower temperatures and higher temperatures. One can clearly see that the minimum time to achieve 100% disinfection depends strongly on the wood type and on the initial temperature. For example, at the initial temperature of −20 °C, this time is 262, 306 and 380 s for trembling aspen, white birch and sugar maple,

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Fig. 10 a Temperature profiles distribution and views of the temperature distribution at half-planes of symmetry of b longitudinal, c radial and d tangential direction for sugar maple sample exposed to microwave at time = 200 s, I0 = 1 W/cm2 , f = 2466 MHz, T0 = −20°C

respectively. At + 20 °C, this time is 101 s, 106 s and 132 s for trembling aspen, white birch and sugar maple, respectively. As a note, the example presented in this chapter concerns the numerical estimation of the phytosanitary treatment time, using microwave energy, for three types of Canadian wood for a simple geometry. However, the approach proposed to model the heat equation in terms of anisotropic enthalpy of volume is, in our opinion, very robust and applicable, a priori, to any type of wood product (frozen or unfrozen). However, in cases where the product to be treated by microwaves is a multi-material with multi-orientation of the thermal conductivity matrices, it is then appropriate to use more appropriate approaches such as the one presented in (Erchiqui and Annasabi 2019). Moreover, this new approach, coupled with Maxwell’s equations, is under development.

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Fig. 11 a Temperature profiles distribution and views of the temperature distribution at half-planes of symmetry of b longitudinal, c radial and d tangential direction.for white birch sample exposed to microwave at time = 200 s, I0 = 1 W/cm2 , f = 2466 MHz, T0 = −20 °C Table 3 Density (MC = 131%, at ambient temperature)

Hardwood Trembling aspen (TA)

Density (kg/m3 ) 808

White birch (WB)

1109

Sugar maple (SM)

1264

9 Conclusion This chapter describes a numerical approach, based on the 3D finite element method, to estimate the time required to disinfect wood products (frozen or unfrozen). For this, the heat conduction equation in terms of anisotropic volume enthalpy is considered. The study concerns the estimation of the optimal time for the phytosanitary treatment of three species of Canadian wood by microwave in accordance with FAO

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Fig. 12 Heat treatment by microwave energy (I0 = 1 W/cm2 , f = 2466 MHz): Evolution of percentage sapwood disinfected (initial temperature is –15 °C)

Fig. 13 Heat treatment by microwave energy (I0 = 1 W/cm2 , f = 2466 MHz): Evolution of percentage sapwood disinfected (initial temperature is 0 °C)

International Standard No. 15. The dielectric and thermophysical properties are a function of temperature, moisture content and orientation of the structure. From this modelling study, it was possible to accurately estimate the time required to reach the temperature recommended by the FAO International Standard. This makes it possible to optimize the time and thus the energy of the microwave heat treatment.

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Fig. 14 Heat treatment by microwave energy (I0 = 1 W/cm2 , f = 2466 MHz): Evolution of percentage sapwood disinfected (initial temperature is 15 °C)

Fig. 15 Effect of initial temperature on critical time* of heat treatment by microwave (MC = 131%, lon , t rad , t tan SG = 0.32). *Time required for 100% disinfection. It can also defined as: tcrt = min tcrt crt crt

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More general cases will be studied in future work to establish the relationship between the treatment time, the initial temperature, the geometry of the sample wood and the physical properties of the wood.

Appendix Electric Field in Parallel with One of the Three Principal Directions of the Wood Rotation of the electric field vector E on 180° does not change the dielectric properties of wood materials (Vyacheslav Komarov 2012). That is, L, R and T are the principal axes of anisotropy and the tensor ε in Eq. (8) may be simplified as ⎡

⎤ ⎡  ⎤ 0 0 εL 0 0 ε L − jε L ⎦ ε = ⎣ 0 εR 0 ⎦ = ⎣ 0 0 ε R − jε R   0 0 εT 0 0 ε T − jε T

(60)

When E is arbitrarily oriented in space and forms an angle θ1 with L, angle θ2 with R, and angle θ3 with T, closed-form expressions for calculation of the relative dielectric constant, ε , and dielectric loss tangent, tan δ, is derived in (Vyacheslav Komarov 2012)   ε = ε L cos2 θ1 + ε R cos2 θ2 + ε T cos2 θ3

(61)

tan δ = tan δL cos2 θ1 + tan δR cos2 θ2 + tan δT cos2 θ3

(62)

with tan δL =

ε L ε R ε T , tanδ = , tan δ = , R T ε L ε R ε T

(63)

If E is parallel to one of the three principal directions of the sample wood (θi = 0, for i = 1, 2 or 3), the power dissipated per unit volume, given by Eq. (11), may be simplified as: Pwave = −Re(∇ · S) =

ω  2 ε0 ε d Ed 2

εd is the dielectric constant in the principal direction d (L, R or T).

(64)

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Synthesis, Characterization, in Vitro Biocompatibility and Antibacterial Properties Study of Nanocomposite Materials Che Wan Sharifah Robiah Mohamad

Abstract Nanocomposites are divided into 3 main types namely metals, ceramics and polymers. Apart from that, there are various methods in the production of nanoceramic, namely mechanical, vapor, high temperature and gel solution. Normally, gel solution was use for low cost and easy for handling. Characterization of nanocomposite can be done using various methods either by use the reaction of electrons, ions or radiation. Each characterization will give different features of nanocomposite either in term or surface, element or other characteristic of nanocomposite. Apart from that, nanocomposite is one of the biomaterial materials suitable for use in vivo or even in vitro and also has antibacterial properties. All this factor will enhance the suitability of nanocomposite to be use in engineering field and other related industry. Keywords Nanocomposite · Synthesis · Characterization · Biocompability and antibacteria

1 Nanocomposites A nanocomposite is a multiple solid material where one of the phases has one, two or more dimension which is less than 100 nm or the structure was having nano-scale repetitive distance between the different phases that make up the material. Nanocomposites are defined as a promising class of hybrid materials that are mixtures of polymers such as polysaccharides and polylactic-co-glycolic acid (PLGA) with inorganic solids at the nanometric scale. Currently, three broad type of nanocomposites based on their matrix: metal matrix nano composites (MMNC), ceramic matrix nano composites (CMNC) and polymer matrix nano composites (PMNC) (Camargo and Wypych 2009). Nanocomposites are significant to upcoming materials which shows the great changes in all the industrial field. This type of material is also going to be a economical barrier for developing country as a once of medical nanotechnology. C. W. S. Robiah Mohamad (B) Faculty of Electronic Engineering Technology, Universiti Malaysia Perlis (UniMAP), Arau, Perlis, Malaysia e-mail: [email protected] © Springer Nature Singapore Pte Ltd. 2021 M. T. Hameed Sultan et al. (eds.), Biocomposite Materials, Composites Science and Technology, https://doi.org/10.1007/978-981-33-4091-6_11

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1.1 Metal Matrix Nano Composites (MMNC) MMNC consist of a metal matrix filled with nano-particle producing remarkable physical and mechanical properties when compared to those of the matrix. This type of nanocomposite (MMNC) will has high strength in shear/compression process, high electrical and thermal stability, wear and chemical resistance. However, the drawback of MMNC is such as highly corrosive and the example of MMNC are Fe–Cr/Al2 O3 , Fe/MgO, Mg/CNT and etc.

1.2 Ceramic Matrix Nano Composites (CMNC) The CMNC consist of a ceramic matrix filled with nanofiller as a metal/nonmetal/ceramic material mostly resulting in improved optical, electrical and magnetic properties as well as tribological, corrosion-resistance and other protective properties. Other than that, CMNC also will enhanced mechanical properties including fracture toughness, stiffness and strength to certain composite. Just like a MMNC, this type of ceramic nanocomposites is low toughness and brittle. The example of CMNC are Al2 O3 /SiO2 , SiC/C, Al2 O3 /CNT and etc.

1.3 Polymer Matrix Nano Composites (PMNC) Inorganic or organic nanofiller are incorporated within polymeric matrix to obtain improved properties like impressive and flexural mechanical properties, light weight, electrical conductivity and ductility. This type of PMNC will increases heat and impact resistance, flame retardancy, mechanical strength and decreases gas permeability with respect to certain types of gases such as oxygen, carbon dioxide and water vapour. The disadvantages of PMNC such as low modulus and strength. The example of PMNC are polyester/TiO2 , polymer/CNT, polymer/latered double hydroxides and etc.

2 Synthesis of Nanocomposites A nanocomposites synthesis is varied from a different solid matric, which a combination of continuous and discontinuous phase with a nano-scale dimension.

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2.1 Mechanical Technique Nanocomposites were synthesized by the solid-state powder processing technique of mechanical alloying in Al–Al2 O3 , TiAl–Ti5 Si3 , and MoSi2 –Si3 N4 systems. The mechanically alloyed powders were consolidated to full density by techniques such as vacuum hot pressing, hot isostatic pressing, and combinations of material. Basically, a synthesis of MMNC were consolidated with Rapid solidification (RSP) (Rohatgi and Schultz 2007), Vapour technique (PVD, CVD), Electrodeposition and chemical method (colloidal and sol–gel processes) and ball milling. However, a different technique will be used for CMNC, which is conventional powder method, polymer precursor route, vapour technique (PVD, CVD), Chemical methods (sol–gel process, colloidal and precipitation approaches, template synthesis) (Camargo and Wypych 2009).

2.2 Enzymatic and Chemical Treatment Technique Synthesis of nanocomposites using chemical and enzyme treatment techniques involves catalysis reactions and the absorption properties of nanoparticles involved in either metal, ceramic, polymer or biopolymer. Generally, on irradiation with UV light, these oxides generated holes and released electrons, which reacted with H2 O and O2 molecules, and adhered on the surface. In result, it produces highly reactive oxygen species (ROS) such as peroxides, superoxide, hydroxyl radicals, and singlet oxygen. ROS are capable to degrade organic water pollutants like dyes efficiently (Brillas and Martínez-Huitle 2015). Generation of ROS also has antibacterial effect due to oxydative stress in an aqueous condition, for example, the hydroxyl radical is responsible for inactivation of Escherichia coli Han et al. 2016. Techniques often used for nanocomposite synthesis using Enzymatic and Chemical treatment technique for MMNC is Colloidal and sol–gel processes. The synthesis of CMNC by use template synthesis, precipitation approaches, Colloidal and sol– gel processes, while for PMNC is intercalation of the polymer or prepolymer from solution, In-situ intercalative polymerization, melt intercalation, direct mixture of polymer and polymer and sol–gel process (Camargo and Wypych 2009). Biocomposites refers to those composites that can be employed in bioengineering field and biofiber (such as wheat, kenaf, hemp, jute, sisal and flax) are one of major component of biocomposites (Usha 2017). Nowadays, the production of biocomposite materials from natural materials (such as plant base, shell, grain) as a new material with improved performance over individual constituent material. Polymers used in the field of health sciences are classified as biopolymers (or biomedical polymers) because of their use in biological systems. These materials are used in biomedical applications such as pharmaceutical, medical device coatings, and resorbable implants that require biocompatibility and nontoxicity (Rudin and Choi 2013).

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3 Characterization of Nanocomposites The nanocomposites were characterized by surface analysis and some other technique to evaluates their properties. The major issue for characterization is to describe the distinctive character of nanocomposite by use external technique to probe into inside structure and properties of material, included nanocomposites element composition, molecular weight and etc.

3.1 Surface Analysis Nanocomposite surface would not stand alone and this surface were interacted with other material or ambient.

3.1.1

Electron-Beam Techniques (Auger-Electron Spectroscopy)

Auger-Electron Spectroscopy (AES) is a widely used surface analysis technique with several applications across industries including medical device, electronics, hard drive, and other general manufacturing segments. It’s basically can be used as a valence—band spectroscopy that is surface sensitive. Observed changes in Auger line shapes have been useful in identifying chemical states of elements at surfaces, particularly carbon, sulfur, nitrogen, and oxygen (Ashok 2016). They are tree basic principle of AES, which is ionization of incident electron transfer of its energy to an inner-shell electron, de-excitation of electron and emission. A basic application of AES is to determine atomic concentration, elemental, chemical deep profiling, chemical imaging, small feature analysis and etc. The AES have their pros and cons, which means selecting the right testing method is critical for achieving the results we desire such as sample size, sample compatibility, type of material, and other factors. In term of advantages, this technique allow for in-depth surface analysis of conductive and semi-conductive surface because of this technique is able to reveal the composition of a surface. Other than that, the ability of AES analysis to focus the electron beam in small area surface such as a small diameter (1–2 μm) material and to enhanced the accuracy and turnaround of the sample (Auger Electron Spectroscopy). However, AES is not suitable to test a polymer, glass or other insulation material and will cause a result in surface damage for this type of material. To reduce a surface damage, a special technique needs to used and element like hydrogen and helium can’t detected because of they have less than 3 electrons. A quantification of is not easy and AES also associated with a relative error rate of 10% in repetitive analysis.

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Ion-Beam Technology (IBT)

An ion beam is a type of charged particle beam consisting of ions, generated. Generated and transported under high vacuum conditions in order to avoid beam degradation by collisions with air molecules. In addition, the reaction products used for analysis are also absorbed or scattered by the presence of air, so the vast majority of IBA takes place in a high vacuum chamber (Grime 2017). Ion-beam technology (IBT) have a several type of reflection such as Secondary Ion Mass Spectroscopy (SIMS), Rutherford Backscattering Spectroscopy (RBS), Hydrogen Forward Scattering Spectroscopy (HFSS) in term of their Ion Beam Analysis (IBA) as shown in the Fig. 1. IBA works on the basis that ion-atom interactions are produced by the introduction of ions to the sample being tested. Major interactions result in the emission of products that enable information regarding the number, type, distribution and structural arrangement of atoms to be collected. IBA can be used for all types of material, provides outstanding uniformity, versatility, and independent ion beam energy. This factor was contributed to huge applications of IBA especially for quantitative analysis of thin film based on their thickness, type of composition, uniformity in depth, solid state reactions and interdiffusion. IBA also use to determines the quantitative measurements of impurities in substrate.

Fig. 1 The type of reflection for Ion-beam technology (IBT) (Muller and Vervoort 2020)

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Other than that, this IBA detect a defect distribution in crystal sample and so on. This feature was enhanced the application type of IBA in nanocomposite study especially in surface analysis.

3.1.3

X-ray Technique

X-ray techniques commonly used for the analysis of materials in engineering field and structural components. These techniques included X-ray fluorescence (XRF) spectrometry, X-Ray Photoelectron Spectroscopy, and X-ray diffraction (XRD). These analytical techniques provide qualitative and quantitative information on the composition and structure of materials with precision (Igwebike-Ossi 2017). The generating of X-rays is important in this technique. Normally the production of X-rays thru electromagnetic radiation (λ, f, E), x-ray radiation and x-ray generation. A basic principle of x-ray technique is ionization/excitation/transfer, de-excitation/relaxation and emission. The x-ray technology, its divide to two type of x-ray scattering, coherent (without a loss in energy) and incoherent scattering (incident x-ray loses some of its energy to the scattering electron) (Fig. 2). The advantages of using x-rays in analysis are x-rays is the cheapest and most convenient method, x-rays are not absorbed very much by air, so the specimen need not be in an evacuate ed chamber (Igwebike-Ossi 2017). The disadvantage is that they do not interact very strongly with lighter elements, so this could impose a limitation on the elements detectable by X-ray techniques.

Fig. 2 Example of X-ray diffraction (XRD) result (Bashir and Liu 2015)

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Fig. 3 FTIR principle (John 2018)

3.2 Fourier Transform Infrared Nanomaterials (FTIR) Fourier transform infrared spectroscopy (FTIR) is a largely used technique to identify the functional groups in the materials (gas, liquid, and solid) by using the beam of infrared radiations. This technique has been considered to be one of the most effective techniques to study and understand the chemical and surface chemistry in various types of membrane (Mudunkotuwa et al. 2014). This type of spectrometer is an optical instrument used to measure properties of light over a specific portion of the electromagnetic spectrum, 5 microns to 20 microns. The advantages of FTIR in engineering and material analysis is this type of spectroscopy can make an identification of inorganic compounds and organic compounds. The speed of all frequencies is measured simultaneously less than 10 min. Other than that, the sensitivity of FTIR is dramatically improved with their detector and high optical throughput which result in much lower noise level. FTIR also easy to handle and suitable in remote sensing with mechanical simplicity. Figure 3 shows the basic principle of FTIR as their light beam split into two by the beam splitter which reflected from the moving mirror and fixed mirror, before being recombined by the beam splitter. The movements of the optical path difference based on fixed mirror changes, such that the phase difference changes with time (Clayborne and Morris 2017).

3.3 Photoluminescence (PL) Spectroscopy Photoluminescence (PL) spectroscopy is a form of light emission spectroscopy in which the light emission comes from a process called photo-excitation. As the light is directed onto a sample, the electrons within the material move into excited

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states. (excitation) When the electrons come down from the excited states to their equilibrium states, the energy can be released in the form of light (Zhang 2018). The PL spectroscopy is a non-contact, nondestructive method of probing materials. It is one of many forms of luminescence (light emission) and is initiated by photoexcitation (i.e. photons that excite electrons to a higher energy level in an atom). Following excitation, various relaxation processes typically occur in which other photons are re-radiated. Time periods between absorption and emission may vary and it depends on type of nanomaterial which are up to milliseconds for Phosphorescence processes in molecular systems; and under special circumstances delay of emission may even span to minutes or hours (Hayes and Deveaud 2002).

3.4 Electron Microscopy An Electron microscope is a microscope that uses a beam of accelerated electrons as a source for visualization. This type of microscope having a high resolution of image (up to 2 million times), able to magnify objects in nanometers, 3D shape of objects with a greater depth of field (Scanning Electron Microscopy). Types of an electron microscopy is Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), Field Emission Scanning Electron Microscopy (FESEM). Advantages of electron microscope is very high magnification (up to 2 million times), incredibly high resolution, material rarely distorted by resolution and diverse application from engineering field, biology and others related industry. However, this electron microscope has a range of disadvantages such as extremely expensive, sample is necessary to coat with very thin layer of metal (such as gold or platinum), sample must be completely dry and energy of the electron beam is very high (Kalsoom et al. 2018).

4 In Vitro Biocompability of Nanocomposites Biocompatibility is related to the behavior of biomaterials in various contexts. The term refers to the ability of a material to perform with an appropriate host response in a specific situation (Jonathan Black 2006). A measurement of biological reactions and parameters assessing the possibility of using certain materials have been gathered in International Standard ISO 10,993 (PN-EN ISO10993-5:2009 2009) (or other similar standards) to determine if a certain material (or rather biomedical product) is biocompatible. Since the immune response and repair functions in the body are so complicated it is not adequate to describe the biocompatibility of a single material in relation to a single cell type or tissue (Medical Device Technology 2008). By considering all of the nanocomposite features, during the development of a nanocomposite, all properties of material require to improve from the starting point of synthesis until the end of an experiment. The addition of even a small

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concentration or amount of nanoparticles (NPs) to the polymer matrix and then adding the nanocomposite to the body will affect the interaction of material with cells. Interaction with immune cells may result in molecular reactions, which may lead to higher sensitivity to disease and cancer growth and also cause undesired allergic reactions to human body (Kononenko et al. 2015; Hirai et al. 2014; Immunol 2014; Radomski et al. 2005). Understanding how a nanocomposite reacts with cells is a challenge from a toxicological point of view, but is crucial for future solutions in biomedicine. Cell research including human cell lines may provide information on the biocompatibility of nanomaterials (Visani de Luna et al. 2016). The example of metalcomposite is titanium due to its high biocompatibility, low weight, and ease of use (Woods et al. 2009; Hales et al. 2007).

5 Antibacteria Properties of Nanocomposites Antimicrobial testing can be used for the in vitro investigation of nanocomposites as a potential biomaterial agent. NCCLS (formerly known as the National Committee for Clinical Laboratory Standards) guidelines for nanocomposites of antimicrobial susceptibility testing especially in quality control issues and current methodologies, such as conventional disk diffusion and microbroth dilution tests method (Ginocchio 2002). However, for minimal inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) values for various nanocomposites compound by measuring the effect of decreasing concentration of biochemical in term of inhibition of microbial growth and lowest concentration of an antibacterial agent to kill a specific bacterium, respectively (Holla et al. 2012).

5.1 Diffusion Method The diffusion methods required to evaluated the antibacterial activity of nanocomposites compounds against Gram positive or Gram negative bacterial.

5.1.1

Agar Well Diffusion (AWD) Method

Nowadays, many microbiology laboratories were use Clinical and Laboratory Standards Institute (CLSI) guidelines as an official antimicrobial routine for bacteria and others microorganism testing. The AWD was used to evaluate the antimicrobial activity of nanocomposite especially from plants or microbial composite. This method was considered as a standard test for certain bacterial pathogen like streptococci, H. influenzae, H. parainfluenzae, Neisseria species by using specific culture media such as Mueller–Hinton agar (MHA) medium; various incubation condition

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(30–37 °C) and interpretive criteria for inhibition zones was observed and analyses (CLSI 2012). The basic procedure for ADD methods is involved with inoculation of agar plate with a standardized inoculum of the test microorganism. Then, filter paper discs (about 6–8 mm in diameter), containing the test compound (sample) at a desired concentration, are placed on the agar surface. After that, this Petri dishes are incubated under 37–40 °C for 16–24 h, and the diameters of inhibition growth zones are measured by use calliper or MOTIC software.

5.1.2

Agar Plug Diffusion (APD) Method

Agar plug diffusion methods is often used to determine the antagonism between more than two different type of microorganism (Jiménez-Esquilín and Roane 2005; Elleuch et al. 2010). The APD methods is involved an agar culture of the strain of interest on medium by tight streaks on the plate surface. After certain incubation period, in growing phase, target microbial cells will secrete a metabolite compound which diffuse in the agar medium. Then, after 12–24 h, a target microbial on respective agar is cut aseptically with a sterile cork borer in 2–3 cm, then deposited on the agar surface of another plate previously inoculated by the test microorganism (such as E. coli). Tis APD plate will incubate for a few hours then the appearance of the inhibition zone around the agar plug will be visible (Balouiri et al. 2016).

5.2 Dilution Method The tube dilution test is the standard method for determining levels of resistance to an antibiotic. Serial dilutions of the antibiotic are made in a liquid medium which is inoculated with a standardized number of organisms and incubated for a prescribed time.

5.2.1

Broth Dilution Method

Broth is the most basic methods of antimicrobial susceptibility testing. This method used liquid medium and it is very suitable to test the antimicrobial activity of solid samples like films. McFarland Standards provide a guide to bacterial suspension standardization used for susceptibility screening. The preparation of 0.5 McFarland microbial inoculum for the broth dilution test. Besides, OD600 is used to measure and determine the turbidity or concentration of bacteria growth in a solution. OD600 is refer to the wavelength in nanometres (nm) and it is used to measure the concentration of a solution (Balouiri et al. 2016). Broth microdilution method is reproducible to a single doubling dilution of the real end point of a series of dilution Gondal et al. 2016). Broth dilution and microdilution

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methods are similar and well established. But microdilution method uses smaller amounts of medium, reagents and tested agents. Because of that, the microdilution method is more economical and less laborious than broth dilution method (Golus et al. 2016). This method is the most suitable ones for the determination of MIC values, as they give the possibility of estimating the concentration of the antimicrobial agent tested (Balouiri et al. 2016). The beginning of process, wells are filled with broth containing various concentrations of antibiotics. Then, inoculated with bacteria and incubated overnight. The minimum inhibitory concentration is determined after 12–24 h incubation.

6 Conclusion Nanocomposite is one of new material to enhance matrix properties of material. A different type of matrix, such as MMNC is to increase yield stress, tensile strength and creep resistance of material. Meanwhile, a CMNC to increase fracture toughness of material and PMNC to increase modulus, yield stress, tensile strength and also a creep resistance. With the hope that the presence of antibacterial activity for some nanocomposites such as biocomposites will be able to improve and enhanced the application of this material in the biomedical and engineering sectors.

References Ashok KS (2016) Experimental methodologies for the characterization of nanoparticles, Engineered Nanoparticles Auger Electron Spectroscopy (2020) Physical Electronic Balouiri M, Sadiki M, Ibnsouda SK (2016) Methods for in vitro evaluating antimicrobial activity: a review. J Pharm Anal 6(2):71–79 Bashir S, Liu J, (2015) Nanocharacterization, advanced nanomaterial and their applications in renewable energy Brillas E, Martínez-Huitle CA (2015) Decontamination of wastewaters containing synthetic organic dyes by electrochemical methods. An updated review. Appl Catal B Environ 166–167:603–643 Camargo PHC, Satyanarayana KG, Wypych F (2009) Nanocomposites: synthesis, structure, properties and new application opportunities. Mater Res 12(1):1–39 Clayborne A, Morris V (2017) Fourier transform infrared spectroscopy (FTIR). Physical Chemistry Laboratory CLSI (2012) Performance standards for antimicrobial disk susceptibility tests, approved standard, 7th ed. CLSI document M02-A11. Clinical and laboratory standards institute, 950 West Valley Road, Suite 2500, Wayne, Pennsylvania 19087, USA Elleuch L, Shaaban M, Smaoui S et al (2010) Bioactive secondary metabolites from a new terrestrial Streptomyces sp. TN262, Appl Biochem Biotechnol 162:579–593 Ginocchio CC (2002) Role of NCCLS in antimicrobial susceptibility testing and monitoring. Am J Health, Syst Pharm 59(8 Suppl 3):7–11

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Golus J, Sawicki R, Widelski J, Ginalska G (2016) The agar microdilution method—a new method for antimicrobial susceptibility testing for essential oils and plant extracts. J Appl Microbiol 121(5):1291–1299 Gondal AJ, Hannan A, Saleem S, Arshad MU (2016) Comparative evaluation of broth and agar dilution techniques for antimicrobial susceptibility testing against multidrug resistant pseudomonas aeruginosa. Pak J Zool 48(5):1517–1522 Grime GW (2017) High-energy ion beam analysis. Encyclopedia of spectroscopy and spectrometry (3rd edn) Hales NW, Shakir FA, Saunders JE (2007) Titanium middle-ear prostheses in staged ossiculoplasty: does mass really matter? Am J Otolaryngol-head neck med Surg 28:164–167 Han C, Lalley J, Devi N, Cromer K, Mallikarjuna NN (2016) Titanium dioxide-based antibacterial surfaces for water treatment. Curr Opin Chem Eng 11:46–51 Hayes GR, Deveaud B (2002) Is Luminescence from quantum wells due to excitons? Phys Status Solidi A 190(3):637–640 Hirai T, Yoshioka Y, Ichihashi K, Mori T, Nishijima N, Handa T, Takahashi H, Tsunoda S, Higashisaka K, Tsutsumi Y (2014) Silver nanoparticles induce silver nanoparticle-specific allergic responses Holla G, Yeluri R, Munshi AK (2012) Evaluation of minimum inhibitory and minimum bactericidal concentration of nano-silver base inorganic anti-microbial agent (Novaron(® )) against streptococcus mutans. Contemporary clinical dentistry, vol 3. pp 288–293 Igwebike-Ossi CD (2017) Failure analysis and prevention: X-Ray technique. pp 153–171 Immunol J (2014) 192:118–119; Capasso L, Camatini M, Gualtieri M (2014) Nickel oxide nanoparticles induce inflammation and genotoxic effect in lung epithelial cells. Toxicol Lett 226:28–34 In vitro biocompatibility testing of biomaterials and medical devices, U. Muller, Medical device technology, March 2008 Jonathan Black (2006) Biological performance of materials John (2018) Fourier transform infrared spectroscopy (FTIR) Principle, instrumentation forum Jiménez-Esquilín AE, Roane TM (2005) Antifungal activities of actinomycete strains associated with high-altitude Sagebrush Rhizosphere. J Ind Microbiol Biotechnol 32:378–381 Kalsoom A, Shahid AK, Sher BH, Abdullah MA (2018) Scanning electron microscopy: Principle and application in nanomaterials characterization. Handbook of material characterization. pp 113–145 Kononenko V, Narat M, Drobne D (2015) Nanoparticle interaction with the immune system. Arch Ind Hyg Toxicol 66:97–108 Mudunkotuwa IA, Minshid AAI, Grassian VH (2014) ATR-FTIR spectroscopy as a tool to probe surface adsorption on nanoparticles at the liquid–solid interface in environmentally and biologically relevant media. Analyst 139:870 Muller P, Vervoort J (2020) Secondary ion mass spectrometer PN-EN ISO 10993-5:2009 (2009) Biological evaluation of medical devices Part 5: tests for in vitro cytotoxicity; international organization for standardization: Geneva, Switzerland Radomski A, Jurasz P, Alonso-Escolano D, Drews M, Morandi M, Malinski T, Radomski MW (2005) Nanoparticle-induced platelet aggregation and vascular thrombosis. Br J Pharmacol 146:882–893 Rohatgi PK, Schultz B (2007) Lightweight metal matrix nanocomposites-stretching the boundaries of metal. Mater Matters, 2.4, 16 Rudin A, Choi P (2013) Chapter 13-Biopolymer, The elements of polymer science and engineering (3rd edn) pp 521–525 Scanning Electron Microscopy (2018) Scientific Data Usha GK (2017) Synthesis and characterization of biocomposite material, MSc thesis. National Istitute of Technology, Rourkela

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Visani de Luna L, Mazarin de Moraes A, Consonni S (2016) Comparative in vitro toxicity of a graphene oxide-silver nanocomposite and the pristine counterparts toward macrophages. J Nanobiotechnol Woods O, El Fata F, Saliba I (2009) Ossicular reconstruction: Incus versus universal titanium prosthesis. Auris Nasus Larynx 36:387–392 Zhang YH (2018) Light absorption and photoluminescence (PL) spectroscopy, Chemistry LibreTexts

Enhancement of Local Drug Delivery System Using Different Design of Gentamicin Loaded in Carbonate Apatite Scaffold N. Mamat, M. Mariatti, Z. A. A. Hamid, and B. H. Yahaya

Abstract A combination of bioactive scaffold with drug delivery of therapeutic agents is a great deal to locally treat bone infections. However, controllable drug release behaviours for different types of drug-incorporated scaffold have been not comprehensively compared. In this study, novel technique was proposed with the addition of bioactive agents in microspheres was incorporated into carbonate apatite (CO3 Ap) scaffold. A simple slurry-dipping method by dispersion of 0.8 wt% suspension of gentamicin (GEN)-loaded polylactic acid (PLA) microsphere (GENMS) using an ultrasonic bath was used to coat the scaffold. GENMS was fabricated by double emulsion. This coated scaffold was compared to the GEN coated scaffold without microsphere and uncoated scaffold with direct loading of GEN. It was confirmed that the microsphere coating did not inhibit the apatite growth of the scaffold when immersed in Hank’s Balance Salt Solution for 4 weeks. The drug release profile exhibited the initial burst and sustained drug release could be improved by the presence of GENMS in the coated scaffold. Moreover, the kinetic release study supported the findings of different drug release based on zero-order, firstorder, Higuchi and Korsmeyer–Peppas models. The results showed that drug release mechanisms were diffusion and degradation controlled for scaffold, while for coated scaffolds led to diffusion and degradation of chitosan and microsphere. Rougher surface of the scaffold by the adhered GENMS on the scaffold facilitated cell proliferation. In short, this multifunctional coated bioactive scaffold has the potential to enhance cell attachment and provide local of controlled drug delivery for bone tissue engineering improvement. N. Mamat (B) Faculty of Electronic Engineering Technology, Universiti Malaysia Perlis (UniMAP), Arau, Perlis, Malaysia e-mail: [email protected] M. Mariatti · Z. A. A. Hamid Biomaterials Niche Area Group, School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia, Engineering Campus, 14300 Nibong Tebal, Pulau Pinang, Malaysia B. H. Yahaya Regenerative Medicine Cluster, Advanced Medical and Dental Institute (AMDI), Universiti Sains Malaysia, Kepala Batas, Pulau Pinang, Bertam, Malaysia © Springer Nature Singapore Pte Ltd. 2021 M. T. Hameed Sultan et al. (eds.), Biocomposite Materials, Composites Science and Technology, https://doi.org/10.1007/978-981-33-4091-6_12

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Keywords carbonate apatite · Scaffold · Microsphere · Drug delivery · Chitosan · Coating

1 Introduction One of the key components for successful bone regeneration is the presence of functional bone scaffold with loadings of drug to promote tissue regeneration. The scaffolds have been used as drug delivery such as therapeutic agents (i.e. antibiotic and anti-inflammatory) to enhance the healing process of the bone. Calcium phosphate (CaP)-based bioceramics such as hydroxyapatite [(Ca10 (PO4 )6 (OH)2 ), HA] is known for its excellent biocompatibility to be used as porous bone scaffold. The apatite found in human bone is consisted by other ions mainly carbonate (CO2− 3 ) content and traces of Na+ , Mg2+ , Fe2+ , Cl− and F− (Ishikawa 2010). Although the presence of these ions are low, they play an important role in the biochemical reactions of bone metabolism. As a result, CO2− 3 containing HA as carbonate apatite (CO3 Ap) has gained much attention due to its chemical composition being closer to bone mineral and favourable for bone growth. Mimicking bone scaffold to natural bone by chemical composition and improved compressive strength is beneficial in tissue engineering. However, the occurrence of bone infection is a destructive problem which leads to post-operative problems and critically need attention. The current therapy to prevent implant associated infection relies on systemically administrated antibiotics. This conventional treatment poses several major disadvantages. This therapy is difficult to achieve the effective localisation of antibiotics and ineffective once the bacteria anchored and synthesise a biofilm on the surface of implant (Yazdimamaghani et al. (2014)). A very high concentration of a drug in bloodstream and other organs easily produce potential side-effects in the body system. Thus, scaffold loaded with antibiotics can act as a platform for local drug delivery additionally used for bone repair defects while enhance cell proliferation. Generally, scaffold with drugs directly applied on the surface have very limited loading capacity with the drug diffusion is quite fast. Consequently, this system is not beneficial for certain osteoarticular therapy (Pei et al. 2018). The option of incorporating drugs in the polymer coated scaffolds is attractive as this approach can lead to a more controlled biomolecule release profile during local delivery of therapeutic drugs (Yao et al. 2013; Meng et al. 2013; Araújo et al. 2017). This is supported by an early study by Kim et al. (Kim et al. 2005) that show the homogeneous entrapment of the drug throughout the 3D scaffold structure is possible and the drug release period can be prolonged. Recent developments concern on the use of microspheres as drug carriers in bone scaffolds. Scaffold fabrication can be formed by incorporating microspheres as a targeted drug delivery system (Francis et al. 2010; Wang et al. 2017). These allow the controlled released of drug and decreased the initial burst of the drug.

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Hence, localized delivery of therapeutic substances can reduce the amount of bioactive compounds and release intracellularly in a prolonging time. The incorporated drug-loaded microspheres can offer site-specific delivery of drugs and controlledrelease of drugs with distinct release kinetics. Drug-loaded microspheres on the scaffold reduces the side effects and improves the efficiency as it uses a significantly less amount of due to the localized release (Gbureck et al. 2008). The strategy of incorporating polymer microspheres into 3D scaffolds have been attracted increasingly attention in recent years. However, fabrication and characterization of porous CO3 Ap scaffold loaded with gentamicin (GEN) in chitosan supported with kinetic release properties has not been reported elsewhere. GEN is aminoglycoside antibiotic fulfils some of the criteria for localized application because of its tissue capability, broad- spectrum bactericidal activity and has no deleterious effect on healing process. The objective of this study was to develop GEN loaded CO3 Ap scaffolds. GEN-loaded polylactic acid (PLA) microspheres (GENMS) was formulated and then incorporated to the fabricated CO3 Ap scaffold. Different drug loaded CO3 Ap scaffolds were fabricated and characterized based on their bioactivity, drug release behaviours and cell viability. This study provides a novel method in coating system for controlling drug release of post-operative implantation procedures.

2 Materials and Methods 2.1 Materials β-TCP (Ca3 (PO4 )2, with grade purum p.a, >96.0%) powder was used as a main component to prepare porous β-TCP scaffold. Polyethylene-imine (PEI, 50 w/v%) was used as a dispersant in β-TCP slurry. Sodium dodecyl sulphate (SDS) powder was used as a pore forming agent while polyvinyl alcohol (PVA) was used as a binder in the ceramic slurry to bind the calcium phosphate particles. Polyethylene glycol (PEG) in liquid form with a specified Mw of 600 g/mol was used as a plasticizer. Denacol EX-622 (aliphatic sorbitol glycidyl ether epoxy) was used as a gelling agent and crosslinker in β-TCP slurry. Distilled water was used as a liquid medium. For transforming β-TCP scaffold to CO3 Ap scaffold, disodium carbonate (Na2 CO3 ) solution was used as a carbonate (CO2− 3 ) source. Medium molecular weight of chitosan (CS, 75–85% deacetylation) was used as a natural polymer solution for coating purpose. In the fabrication of drug-loaded microsphere, PLA microsphere was fabricated by PLA pellet. Food grade PLA has obtained Food and Drug Administration (FDA) approval. Dichloromethane (DCM) with >99.5% purity was purchased from Merck Millipore and PVA (80% hydrolysis) was purchased from Sigma Aldrich. GEN reagent solution with 10 mg/ml was used as encapsulated drug. For bioactivity test, Hanks’ Balanced Salt Solution (HBSS) was used as a supporting solution to investigate the apatite formation.

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3 Methods 3.1 Fabrication of GEN Loaded PLA Microsphere Scaffolds were fabricated using gelate-freeze casting method which is a similar method as described in our previous work (Darus et al. 2018). Fabrication of CO3 Ap scaffold was done using hydrothermal treatment method. Details of the method was described in our previous study (Mamat et al. 2017). The results were based on 5 days hydrothermal treatment in 5 molar Na2 CO3 .

3.2 Fabrication of GEN Loaded PLA Microsphere GEN-loaded PLA microsphere was fabricated using double emulsion solvent evaporation (ESE). In this study, volume ratio of dispersed phase to continuous phase, 1:3 (PLA: PVA) was used to fabricate PLA microsphere. Firstly, 9 wt.% of PLA pellets was dissolved in 30 ml DCM and followed by dispersion of 1 ml GEN solution (with concentration, 10 mg/ml or 10,000 ppm). This solution is subjected to vigorous homogenization to yield the primary emulsion. Then the primary emulsion was immediately emulsified into 90 ml PVA solution. The mixtures were stirred at ~1250 rpm for 3 min to form secondary emulsion at room temperature. Then, the speed of the stirrer was decreased to ~250 rpm for overnight to allow the evaporation of DCM. The particles of the PLA were formed at the bottom of the flask and was washed, filtered and dried overnight at room temperature before they were collected. In order to determine the encapsulation efficiency, PLA microspheres loaded with gentamicin at weight 40 mg were fully degraded in 5 ml of 1 M NaOH solution. The samples were left for overnight until it was degraded and ultraviolet (UV) visible spectroscopy was conducted to determine the encapsulation efficiency (EE%). Scanning of absorbance intensity of GEN was detected at 195 nm wavelength. The calculation of EE% and DL% of the GEN were calculated using Equs. 1–3: Encapsulated drug mass(mg) =

Concentration from data(ppm) × 1ml 1000

Inital drug mass (mg) = Encapsulation Efficiency, EE(%) =

10mg × 1ml ml

Encapsulated drug mass(mg) × 100 Initial drug mass

(1) (2) (3)

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3.3 Incorporation GEN into Carbonate Apatite Scaffold In order to incorporate GEN in the uncoated scaffolds, 2% of GEN solution was prepared by dissolving 0.3 ml (10 mg/ml) of GEN in distilled water at room temperature. The formulation was modified based on the previous work reported by Shah et al. (Shah et al. 2016) by using similar drug. After that, cylindrical shape of CO3 Ap scaffold was immersed in GEN solution for 4 h in vacuum to allow complete infiltration of the drug. The immersed CO3 Ap scaffold was then dried in vacuum oven at 80 °C for 24 h.

3.4 Process on CO3 Ap Scaffold with GEN and GENMS in CS Solution In order to coat CO3 Ap scaffold with CS containing GEN, GEN was mixed with 0.3 ml of 2% solution (10 mg/ml) in 0.5 wt% CS solution. Firstly, an aqueous suspension of GENMS (0.8 wt%) was mixed in 0.5 wt% CS solution, the formulation was used according to the previous work (Francis et al. 2010). The GENMS suspension in CS was sonicated using an ultrasonic bath (Transsonic TP690) for 15 min to improve the dispersion of the microspheres by separating possible microsphere agglomerations. After both coating solutions were ready, CO3 Ap scaffold with 0.45 g in cylindrical shape with dimension of about 10 mm (diameter) × 2 mm (height) was immersed in the solutions for 4 h in vacuum to allow complete infiltration. The immersed CO3 Ap scaffold was then dried in vacuum oven at 80 °C for 24 h. The scaffolds were weighed before and after the coating process and the amount of GENMS loaded and CS coated on the scaffolds were estimated from the weight change.

4 Characterizations 4.1 Scanning Electron Microscope Scanning electron microscope (SEM) (Zeiss Supra 55VP, Germany) was used to analyse the morphology of scaffolds and PLA microsphere. Cross-section of porous scaffolds was observed, and pore size were recorded. The morphologies of the sample were captured using secondary mode at an accelerating voltage of 5 kV.

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4.2 Bioactivity Test GEN incorporated CO3 Ap scaffolds in the size of 10 mm (diameter) × 2 mm (height) were soaked in HBSS for 28 days. Each scaffold was soaked in 50 ml of HBSS and kept at 36.5 ± 1 °C for the set period. After immersion in HBSS, the scaffolds were removed, rinsed gently with deionized water, and dried in an oven at 50 °C for 24 h.

4.3 In Vitro Cell Proliferation Cell study on the samples was carried out with 0.15 g in cylindrical shape. The samples were decontaminated by soaking them 3 times in 70% ethanol for 30 min. After that, the samples were immersed in PBS three times for 15 min each, before being left to dry in sterilized hood for 24 h. Prior to cell seeding, the samples were immersed in Dulbecco’s modified Eagle’s medium (DMEM) for 30 min. The hFOB 1.19 cells (Homo sapiens, CRL-11372TM, ATCC, Manassas, VA, USA) were cultured in Dulbecco’s modified Eagle’s medium (DMEM, Gibco Life Technologies)) and supplemented with 10% fetal bovine serum (FBS, Gibco Life Technologies), 1% antibiotic–antimycotic (AA, Gibco Life Technologies). The cells in culture flask (T-25) multiplied in an incubator with humidified atmosphere of 5% CO2 at 37 °C. Cell proliferation was assessed after 1, 2, 3, 5 and 7 days using PrestoBlue viability reagent (Molecular Probe, Life Technologies). PrestoBlue was added in an amount equal to 10% of the volume in the well. The well was then incubated for 20 min prior to absorbance detection at 570 nm using microplate spectrophotometer (Multiskan Spectrum). The absorbance data were analysed according to formulation provided by manufacturer’s instruction (Molecular Probe, Life Technologies).

4.4 Drug Release Test in Vitro In order to determine the GEN release profile, the uncoated and coated CO3 Ap scaffolds were individually immersed in glass vials containing 10 ml and 0.1 M phosphate buffered saline (PBS) solution. The glass vial placed in shaker with 60 rpm at constant temperature of 37 °C. At pre-determined time intervals, aliquots of 3 ml from each sample were extracted and replenished with fresh PBS solution to maintain the total volume of 10 ml. The drug release study was performed in triplicate for up to 28 days for each group. The level of GEN in the elution was detected by UV–Vis spectrophotometer at a wavelength of 195 nm.

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4.5 Degradation Rate The in vitro biodegradation of the scaffolds was studied by calculating the weight loss the scaffolds over the time. The dry weights after immersion in HBSS were noted as wt . The percentage weight loss with respect to the initial weight (wi ) was calculated based on Eq. 4 (Olad and Farshi Azhar 2014): Weight loss =

wi− wt × 100% wi

(4)

4.6 Drug Release Kinetics The release data of GEN were mathematically fitted to four different kinetic models to fit the results of the in vitro release data; zero-order, first-order, Higuchi and Korsmeyer–Peppas models. These models are often utilized to designate the drug release mechanism from the microsphere (El-Say 2016; Unagolla and Jayasuriya 2018) while Peppas model was the most commonly used model for a mathematical description of drug delivery kinetics and it is often applied to porous ceramic delivery systems (Gbureck et al. 2008). From the linear regression of these plots the correlation coefficient (R2 ) values were calculated and compared. Equations 5–8 show the different release kinetic models; Zero-order kinetic : Qt = Q0 + K0 t

(5)

First-order kinetic : ln Qt = ln Q0 + K1 t

(6)

Higuchi s model : Qt = KH√t

(7)

Korsmeyer-Peppas model : Mt /M∞ , (Qt ) = ktn

(8)

where Q0 is the initial amount of drug in solution (it is usually zero), Qt is the amount of drug released in time t, K0 is the zero-order release constant, K1 is the first-order release constant, KH is the Higuchi dissolution constant, Mt /M∞ is the fraction of drug released at time t, k is the rate constant, and n is the release exponent of the Korsmeyer-Peppas model. Korsmeyer-Peppas equation was conducted to further confirm that both diffusion and erosion mechanism were involved in releasing the drug by calculating the values of drug release exponent (n). The interpretation of the diffusional constant n for the samples was referred according to Table 1.

328 Table 1 Categories of release mechanism depending on sample geometry and the release exponent n of the Peppas model (Hess et al. 2017)

N. Mamat et al. Release exponent n

Release mechanism

Cylinder

Sphere

n ≤ 0.45

n ≤ 0.43

0.45 < n < 0.89

0.43 < n < 0.85 Anomalous (non-Fickian) transport

n = 0.89

n = 0.85

Case-II transport

n > 0.89

n > 0.85

Super case-II transport

Fickian diffusion

5 Results and Discussion 5.1 GEN-Loaded PLA Microsphere, Encapsulation Efficiency and Microstructures Figure 1 shows the bulk shape and microstructure of GENMS fabricated from double ESE. Almost perfect spherical microspheres with a uniform surface morphology are observed from the microstructures. The percentage of EE and DL of GEN incorporated in GENMS was determined based on standard curve of GEN concentration in the range of 10–400 ppm. From the calculation based on Eq. 1 to Eq. 3, the average of EE of GENMS was 13.97% ± 0.311. Due to GEN is a very hydrophilic drug, it tends to leach out into water phase when microsphere are fabricated using ESE method which probably made the obtained encapsulation is low (Naraharisetti et al. 2005). Figure 2 presents SEM morphology of GEN incorporated into CO3 Ap scaffolds. Figure 2a shows the neat CO3 Ap scaffolds with the pure apatite crystal structure without coating layer. Figure 2b shows the morphology of GEN incorporated directly into the CO3 Ap scaffolds (GEN-CO3 Ap scaffolds). This morphology was similar with neat CO3 Ap scaffolds (Fig. 2a) where only apatite crystal structure was

Fig. 1 Microstructure of GENMS fabricated through double ESE technique. (Images were observed at a 1500× and b 3000 × magnification)

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Fig. 2 SEM images of a neat CO3 Ap scaffolds b GEN-CO3 Ap scaffolds, c CS-GEN-CO3 Ap scaffolds and d CS-GENMS-CO3 Ap scaffolds

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observed. Figure 2c shows the CS-GEN-CO3 Ap scaffolds with a single CS coating layer containing GEN on the apatite structure while Fig. 2d shows the morphology of CS-GENMS-CO3 Ap scaffolds with a mix coating of CS coating layer and GENMS. Both CS-coated CO3 Ap scaffolds (Fig. 2c (i) and (d)(i)) exhibits the retaining pores structure after coating process especially for CS-GENMS-CO3 Ap scaffolds (Fig. 2d) which containing mixed coating layer. It should be noted that pore structures are undistracted as shown in Fig. 2b-d (i) after the coating process. This is to ensure that bone cells could penetrate to the pores of bone scaffold for new bone formation. Therefore, the present study shows that all scaffolds able to maintain their pore size between 100 and 250 μm which is suitable to facilitate cell adhesion and internal mineralized bone formation (Xia et al. 2013).

5.2 Bioactivity Evaluation 5.2.1

Biodegradation

The scaffold acts as a temporary template should be replaced by actual extracellular matrix (ECM). Therefore, the biodegradable behaviour of scaffolds is a great importance because it provides structural support to the newly formed tissue (Swain and Bhattacharyya 2013). Figure 3 shows the biodegradation of scaffolds as a function of immersion time in HBSS at 37 °C for 4 weeks. For day 7 and day 14, degradation rate demonstrated that uncoated scaffold has higher degradation capability (4.6–5.1%) in comparison with coated scaffolds (1–4.4% and 0.8–2.9% for CS-GEN-CO3 Ap and CS-GENMS-CO3 Ap, respectively). This is due to the absence of CS layer delayed the

Fig. 3 Biodegradation of uncoated and coated scaffold for 4 weeks immersion in HBSS at 37 °C

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scaffold degradation. Moreover, presence of GENMS showed the lowest degradation for the coated scaffolds. After day 21 and day 28, CS-GEN-CO3 Ap scaffolds shows slightly lower degradation percentage than GEN-CO3 Ap scaffolds. Since CS has hydrophilicity behaviour (Abd-Khorsand et al. 2017), the coated scaffold with GEN shows a quick hydrolysis of its chains during interaction with HBBS solution. However, the quick degradation was controlled by the additional of GENMS for the coated scaffold of CS-GENMSCO3 Ap scaffolds. This is due to PLA microsphere has –CH3 groups as side chains of a polymer chain and thus PLA surface shows hydrophobicity towards the degradation medium (Dorati et al. 2017; Rakmae et al. 2010). The slow degradation rate of CS-GENMS-CO3 Ap scaffolds may be good, as it can provide a more sufficient time for the newly-formed tissue and the ECM during bone remodeling, while promoting the strength throughout the implantation (Sharma et al. 2016).

5.2.2

Apatite Formation

After 28 days immersion, bonelike apatites were clearly seen on both uncoated and coated scaffolds as shown in Fig. 4. Bonelike apatite grew bigger and fully covered on the surface of GEN-CO3 Ap scaffold (Fig. 4a). The apatite with crystal structure showed flake-like morphology. This precipitation indicated that uncoated scaffold enhanced apatite growth. Figure 4b shows CS-GEN-CO3 Ap scaffold has apatite with the microcracks on the surface. The cracks were formed by shrinkage of the surface layer upon drying after the scaffold being extracted from the HBSS. This is also reported by Francis et al. (Francis et al. 2010) that performed bioactivity of bioglass tablet in SBF solution. Crack development is usually a common morphological feature as a part of the reaction for growing apatite layer, which is typical of bioactive materials (Mancuso et al. 2017). As suggested by previous studies, this behaviour is considered the initial stage of the formation of the apatite layer on scaffold surface (Mancuso et al. 2017; Rahaman et al. 2012). CS-GENMS-CO3 Ap scaffold shows a dense layer of bonelike apatite with globular and cauliflower shape (Fig. 4c). Even though, the scaffold was coated with GENMS, the scaffold surface was still capable to allow the bonelike apatite growth within 28 days. This is due to direct contact with the surrounding fluid facilitated the ion-exchange process in HBSS and leading to the apatite formation (Kokubo et al. 2009; Yamaguchi et al. 2014). Moreover, during the process of apatite formation, some GENMS could have detached because of the apatite structures growing on the surface of the scaffold. Therefore, this effect would expose scaffold surface to the HBSS, in promoting the apatite formation rate. It should be noted that, due to uneven surface of the scaffolds, it is very difficult to produce a microsphere coating that fully covers on the entire surface. The CSGENMS-CO3 Ap has probably some edges that uncovered by the microspheres on the surface. Thus, it provides a potential bioactive surface to facilitate ion exchange and induce apatite formation instead of preventing it.

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Fig. 4 SEM images of bonelike apatite growth after 28 days immersion on a GEN-CO3Ap, b CSGEN-CO3Ap and c CS-GENMS-CO3Ap

6 Drug Release Profile and Kinetics Mechanism Figure 5 reports the cumulative percentage of GEN release profiles from free microspheres of GENMS, uncoated scaffold of GEN-CO3 Ap scaffolds, coated scaffold of CS-GEN-CO3 Ap scaffolds and coated containing GENMS scaffolds of CS-GENMSCO3 Ap scaffolds. The release profile was measured for 28 days of samples in PBS release medium. GENMS and GEN-CO3 Ap scaffolds were used as references to compare the release kinetics to the coated scaffolds. The drug release profile showed three zones; (a) initial burst release period, (b) induction release period and (c) sustained release period (Macías-Andrés et al. 2017). Burst release and sustained cumulative release mechanisms could be observed by all scaffolds. From the whole release profile, it obviously showed a lower percentage cumulative of GEN released from the scaffolds compared to the GENMS at each time point.

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Fig. 5 Cumulative percentage of GEN released in PBS from free GENMS, uncoated and coated scaffolds for a 28 days with zone I is initial release, zone II is induction release and zone III is sustained release. b the enlargement of cumulative GEN release within 24 h

GENMS exhibited a very high initial burst release with 59% of GEN while 18% from GEN-CO3 Ap scaffolds within 4 h. In the same period, both coated scaffolds released GEN with 11% and 9% for CS-GEN-CO3 Ap scaffolds and CS-GENMSCO3 Ap scaffolds, respectively. The scaffolds showed similar release profiles but slight differences in the amount of GEN released. GEN was significantly released from free microsphere at initial stage of zone I. This is shown by sharp slope compared to the release rate shown by those uncoated and coated scaffolds. As can be seen, a secondary burst release occurred by GENMS at approximately day 3. It may be due to the remaining GEN was entrapped deep within the microspheres and comes out only when the microsphere starts to degrade or after water has penetrated sufficiently into the microspheres. Furthermore, much higher drug release rates observed in GENMS was due to much higher surface area to volume shown by GENMS than a scaffold. The possible reason for this burst release

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was also assisted by the hydrophilic nature of the drug itself and related to the surface associated GEN. In the present study, in vitro release experiment carried out in PBS which the dispersed GENMS have their entire surface in contact with the solution. However, for CS-GENMS-CO3 Ap scaffolds only the outermost layer of GENMS on the scaffold were in contact with the PBS solution. In addition, the additional path length of the drug from inner atmosphere to the outer release media required the dissolution and/or degradation of CS layer and/or PLA microsphere to release the GEN. According to second and third zone profiles with a change in the slope of the release curve, the release of GEN from the uncoated scaffold was higher than those of coated scaffolds. Based on cumulative release for day 7, the cumulative were 83, 68 and 52% for GEN-CO3 Ap scaffolds, CS-GENMS-CO3 Ap scaffolds and CSGEN-CO3 Ap scaffolds, respectively. It indicated that, the drug release was slower after the GEN was loaded in the coated scaffolds due to the increasing of dissolution path created by CS layer. In term of coated scaffold, the CS-GEN-CO3 Ap scaffolds and CS-GENMSCO3 Ap scaffolds were specifically compared to examine the role of GENMS within CS-coated scaffold. As can be seen, the GEN released from CS-GENMS-CO3 Ap scaffold was higher than the CS-GEN-CO3 Ap scaffolds. This is due to the presence of GENMS attached to the scaffold surface reduced the path of drug dissolution from inside of scaffold as compared to CS-GEN-CO3 Ap scaffolds. This also proved that the hydrophobic properties of PLA microsphere were not the barrier factor because the GEN was diffused to the media phase due to its hydrophilic nature. Furthermore, degradation of PLA is very slow (Silva et al. 2018) and thus this mechanism can be neglected within the drug release period in this study. Hence, the present study shows that the diffusion rate of the GEN was higher than the rate of microsphere degradation. It can be concluded that GEN release behaviour depends mainly on drug diffusion and not on polymer degradation (Macías-Andrés et al. 2017). It is also noted that, the components PBS of the release medium used in this work are non-solvents of PLA (Macías-Andrés et al. 2017). Therefore, the shrinkage or swelling of the polymer is negligible which results in a diffusion-controlled drug release behaviour. In the last zone, GEN was completely released for 22 days from GENMS than those loaded GEN in scaffolds. It is well known that high initial burst showed by GENMS is usually undesirable because the drug released in this period is not available for prolonged release and more importantly it may cause toxic side effects. Therefore, the results demonstrated that scaffold systems can be used to reduce initial burst release while controlling drug release in the prolonging time. CS coating in the present study seems to be more effective in reducing the initial burst release with a sustained release profile over the time (Yao et al. 2013). This was attributed by the proper dispersion of GEN for both coated scaffolds. As reported elsewhere (Ghorbani et al. 2016; Khodaei et al. 2017), drug, growth factor and genes can be directly mixed into the polymer solution in order to prepare a drug carrier with controlled-release profile. Thus, the result demonstrated that CS-GENMS-CO3 Ap scaffold has the best sustained-release function, which can increase the targeted drug

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release using coated CO3 Ap scaffold. Hence, GENMS plays the role to determine an adequate local drug release at defect site. In order to assess the mechanism of GEN release, the release kinetics for all scaffolds based on in vitro release analysis are presented in Table 2. The release kinetic from GENMS was used as a reference. The release behaviours and related constants of GEN from GENMS and scaffolds were calculated for the zero-order, first-order, Higuchi and Korsmeyer–Peppas model equations. All kinetics models are valid only for the first 60% of drug release (Unagolla and Jayasuriya 2018). Table 2 is used to find the best fit model of the experimental data for each sample type by comparing their correlation coefficient (R2 ) values. The highest values for R2 indicated that GENMS and CS-GENMS-CO3 Ap followed first-order (log cumulative percentage of drug remaining versus time) release kinetic model while GEN-CO3 Ap scaffolds and CS-GEN-CO3 Ap scaffolds followed Higuchi’s release kinetic model (cumulative percentage of drug released versus square root of time). The Higuchi model by GEN-CO3 Ap scaffolds and CSGEN-CO3 Ap scaffolds indicated that the kinetics of GEN release from both scaffold types was governed by Fickian diffusion. It describes the diffusion-controlled release of water-soluble drugs from porous scaffold systems (matrix) with the assumption of perfect sink conditions and negligible polymer swelling or dissolution (Gbureck et al. 2008). However, this model cannot be well applied to swelling or degradable systems with changes in diffusivity (Gbureck et al. 2008). Thus, a decrease in GEN release over time is expected as the drug is depleted in the matrix and diffusion path length is increased. In contrast, the first-order release kinetic model reflects reservoir type drug delivery system (Unagolla and Jayasuriya 2018; Xu et al. 2008). The results showed that both GENMS and CS-GENMS-CO3 Ap scaffolds used microsphere as a main source of GEN to be released out. It also related to the release of GEN from the systems where dissolution rate was dependent on the concentration or dissolving of microsphere and coating layer, respectively. Thus, the release of GEN from these samples was slowed down as the dissolving materials increased (Budiasih et al. 2014). Table 2 Release kinetic parameters calculated from the GEN release data in PBS solution using different mathematical models describing drug release mechanisms (R2 = correlation coefficient; n = diffusional release exponent representing the type of transport) Samples

Zero-order kinetic (Qt = Q0 + K0 t)

First-order kinetic (ln Qt = ln Q0 + K1 t)

Higuchi’s model (Qt = KH√ t)

Kosmeyer-Peppas model (Qt = ktn )

R2

R2

R2

R2

n

GENMS

0.6306

0.8059

0.6913

0.6116

0.7011

GEN-CO3 Ap

0.9400

0.9730

0.9851

0.8687

0.7505

CS-GEN-CO3 Ap

0.8048

0.8742

0.9537

0.9402

0.7704

CS-GENMS-CO3 Ap

0.9722

0.9726

0.9391

0.9237

0.7637

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Fig. 6 Mechanism of GEN release from GENMS

The dissolution data of drug release profiles were further fitted into KorsmeyerPeppas equation (log cumulative percentage of drug released versus log time). The diffusional release exponents (n) were found to be in between 0.7011–0.7704, which attributed to anomalous (non-Fickian) transport for all sample types. According to Gbureck et al. (Gbureck et al. 2008) and Hess et al. (Hess et al. 2017) when n = 0.43 (sphere) or n = 0.45 (cylindrical) the drug release mechanism is governed by the Fickian diffusion, when 0.43 < n < 0.85 (sphere) or 0.45 < n < 0.89 (cylindrical) it is anomalous (non-Fickian) transport and when n = 0.85 (sphere) or n = 0.89 (cylindrical) it is case II transport. This analysis was further confirmed that both diffusion and erosion mechanism involved in releasing the drug from the free GENMS, uncoated and coated scaffolds. The incorporation of GEN into microsphere and the scaffold structure was expected to lead to a hindrance in diffusivity and built storage of drug. It may release the trapped GEN while gradually degrade the microsphere (for GENMS), degrade the microsphere and/or CS (for CS-GENMS-CO3 Ap scaffolds), degrade the scaffold (for GEN-CO3 Ap scaffolds) and degrade the CS and/or CO3 Ap scaffolds (for CSGEN-CO3 Ap scaffolds). Therefore, this mechanism resulted to the observation that the amount of released GEN among these samples were different. In addition, kinetic release mechanism can be in line with the in vitro drug release profile as shown in Fig. 5. Thus, Fig. 6 summarize the release behaviour of GEN in the present study.

7 Cell Proliferation Effective scaffolds for TE application need higher surface area and void spaces for cell activities. Cell proliferation for the GEN incorporated into CO3 Ap scaffolds was assessed using PrestoBlue assay after culturing for 1, 2, 3, 5 and 7 days (Fig. 7). For day 1 until day 7 of incubation, the scaffolds showed higher cell proliferation compared to the control cell (cell without scaffold) with CS-GENMS-CO3 Ap scaffold has the highest among others. It is believed that, the cell proliferation is dependent on the surface properties of the scaffold whereby the CS-GENMS-CO3 Ap scaffold

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Cell proliferation (%)

200

337

Control

CS-CO 3Ap

CS-GEN-CO3Ap

CS-GENMS-CO3Ap

180 160 140 120 100 80 60 40 20 0 1

2

3

5

7

Time (day) Fig. 7 Rate of proliferation of hFOB cells grown on control, CS-CO3 Ap scaffold, CS-GEN-CO3 Ap and CS-GENMS-CO3 Ap scaffold for 1, 2, 3, 5 and 7 days

provided higher surface roughness due to adherence of GENMS on the CO3 Ap scaffold surfaces. This possible behaviour is illustrated in Fig. 8. These results indicated that more hFOBs proliferated to that scaffolds surface due to more surface roughness

Fig. 8 Possible different surface roughness by a CS-GEN-CO3 Ap or GEN-CO3 Ap and b how the GENMS attach and fill up uneven surface of CS-GENMS-CO3 Ap

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that played an important role in enhancing cell attachment, proliferation and bone matrix formation (Ibrahim et al. 2016). The results also show that the incorporation of GEN in bone scaffolds improved in osteoblast function rather than scaffold without containing GEN (Farokhi et al. 2016). In addition, cell viability more than 50% demonstrated that drug-delivery scaffolds has no toxicity to hFOBs cells (Ibrahim et al. 2016). As a result, the CSGENMS-CO3 Ap scaffold has potential application prospects in the infection therapy during bone regeneration.

8 Conclusion This work has led to the development of novel multifunctional CO3 Ap scaffold for successful bone regeneration. Presence of gentamicin-loaded PLA microsphere (GENMS) in the coated scaffold surfaces has controlled drug release rate while exhibiting more surface roughness to increase cell viability. GEN delivery capability from GENMS-CS-CO3 Ap scaffold was supported with kinetics release results. The drug was released in a slow and sustained manner compared to higher burst release measured from GENMS and from uncoated CO3 Ap scaffold (GEN-CO3 Ap scaffold). Moreover, adherence of GENMS on CO3 Ap scaffold was not distracted to the growth of apatite nucleation for 28 days immersion. GENMS-CS-CO3 Ap scaffold developed in the study exhibit adequate surface roughness, enhanced bioactivity, controlled drug delivery function and biocompatibility, being thus of high interest for bone tissue engineering.

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